TiO2 catalyst on anodized aluminum plates for structured catalytic reactors

Thin Solid Films 479 (2005) 64 – 72 www.elsevier.com/locate/tsf

Grafting of VOx /TiO2 catalyst on anodized aluminum plates for structured catalytic reactors Thierry Giornelli, Axel Lffberg*, Elisabeth Bordes-Richard Laboratoire de Catalyse de Lille, UMR 8010, USTL-ENSCL, Baˆt. C3, Cite´ Scientifique, 59655 Villeneuve d’Ascq, France Received 9 March 2004; accepted in revised form 12 November 2004 Available online 24 December 2004

Abstract Structured reactors are promising to carry out exothermic reactions because the heat transfer is better controlled than in usual packed-bed reactors. However the coating by oxide powders which must exhibit catalytic activity/selectivity while being mechanically stable is not so straightforward. We have studied the parameters to be controlled to coat aluminum walls by V2O5/TiO2 catalysts which are used in the mild oxidation of hydrocarbons and NOx abatement. The dip-coating technique using metallic alcoholates has been chosen for the grafting of TiO2 on Al2O3/Al, which is controlled by X-ray Photoelectron Spectroscopy (XPS). A monolayer of TiO2 is first grafted, and then a porous film of TiO2-anatase is deposited by sol–gel. Finally, VOx species are grafted on titania and their loading again determined by XPS. Techniques such as Laser Raman Spectroscopy, Scanning Electron Microscopy are used to characterize the samples after each step, and the porous texture is determined. The layers are mechanically and thermally stable. The dispersion and nature of VOx species on TiO2/Al2O3/Al are similar to what is found in literature for TiO2 powders, showing thereby that the shaping of anatase support on plates has not modified the chemical properties of VOx /TiO2-anatase system. D 2004 Elsevier B.V. All rights reserved. PACS: 68.35.
p; 68.55.
a; 68.55.Nq; 81.15.
z; 82.65.Jv Keywords: Structured reactors; Catalytic walls; Dip-coating; V2O5/TiO2 monolayer; XPS; Catalysis; Coatings; Titanium oxide; Vanadium oxide

1. Introduction In industrial heterogeneous catalysis, three main types of reactors are used: fixed-bed reactors, because of their simple setup, recycle reactors and fluidized-bed reactors which both offer a large internal surface area and high free volume. In spite of their advantages, these reactors suffer from several drawbacks like poor heat transfer coefficient of the catalyst bed for the first one [1,2] or attrition and erosion of the catalyst powder for the others. Microreactor technology [3,4] is expected to have a number of advantages for chemical production compared to fixed-bed reactors, particularly in the case of strongly exothermic reactions like combustion or selective oxidation of hydrocarbons. The greatly reduced pressure drops (up to two orders of * Corresponding author. Tel.: +33 320434527; fax: +33 320436561. E-mail address: [email protected] (A. Lffberg). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.139

magnitude) because of laminar flow in straight channels, and the more favourable heat and mass transfers, are among advantages of catalytic microreactors. For example, by performing the reduction of hydrocoumarin in a microchannel reactor, the yield in the corresponding alcohol could be increased from 43 mol% in a conventional fixed bed reactor to 61 mol%, simply because of the good isothermicity [5,6]:

We have chosen to study the coating of metallic plates with V2O5/TiO2 catalyst which is used in several reactions of interest. It is among the most used catalysts for oxidation of aromatics (e.g., the industrial production of phthalic anhydride from o-xylene) [7–10]. More recently, it was shown to be effective in the Selective Catalytic Reduction of

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NOx [11,12], or even in the selective oxidation of ethane [13–15] or of propane [16]. This catalytic system is a relevant example of the strong influence exerted by an oxidic support on both catalytic properties and solid state reactivity of the active metal oxide [17,18]. Moreover, there is a general agreement that the type and activity of surface species on the dispersed metal oxide depend on the specific metal oxide/support system [19]. Therefore any means used to modify the properties of the vanadia phase by modifying those of the uppermost layer of the support may lead to interesting properties. Apart from an early paper in the 1980s by Chandrasekharan [20] who used V2O5/TiO2 as a catalytic coating for wall reactor, very little has been done with this catalytic system. We have studied the coating of anodized aluminum plates with titanium dioxide (anatase) and the subsequent grafting of the active vanadia phase. The sol–gel method is one of the most appropriate processes to prepare thin oxide coatings, because of several advantages: high homogeneity, easy control of composition, low processing temperature. Large area coatings can be obtained at low equipment cost. In particular, sol–gel methods are efficient in producing thin multicomponent oxide layers on various substrates [21]. Titanium dioxide has been investigated as coating material for optical thin films because it is highly transparent, has high refractive index and is chemically stable [22,23]. Another application concerns photocatalysis [24,25], particularly useful in the decomposition of organic compounds in waste water. Crystalline TiO2 exists as three main forms [26]. Rutile is the most stable of the three and its formation depends on starting material, deposition method and calcination temperature. In particular, it has been shown that TiO2 thin films can evolve from an amorphous phase into crystalline anatase and from anatase into rutile by calcination [27]. In catalysis however the anatase form is preferable because its strong interaction with vanadium oxide allows to generate a molecular dispersion of VOx oxide layer [7,28]. TiO2 supported on Al2O3 has been prepared by different techniques such as chemical vapour deposition [29], precipitation from TiCl4 [30] or impregnation [31], the chosen method strongly influencing the dispersion of TiO2 on Al2O3. However, as compared to that of alumina or silica, the specific surface area of titania supports is generally low. In addition, the anatase form has a poor thermal stability at high temperature. In this work, we have studied the coating of VOx monolayer on TiO2-anatase, itself grafted on porous alumina grown by anodization on Al plate. The characterization of the deposits at various stages of the preparation suffers from several difficulties because of the large contribution of the metallic plate, whereas most of the experimental equipments used in the field of catalysis are designed for powders. In particular, it is impossible to determine precisely the amount of surface hydroxyls on alumina (for TiO2 coating), or on TiO2 (for VOx coating), to be neutralized when grafting

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monolayer deposits. We have extensively used X-ray Photoelectron Spectroscopy (XPS). The structural properties of coated plates differing by Ti/Al and V/Ti compositions have been studied by Scanning Electron Microscopy (SEM), Laser Raman Spectroscopy (LRS), and their texture has been analysed by Brunauer-Emmet-Teller (BET) and Barett-Joyner-Hallenda (BJH) methods. To study the decomposition of precursors, powders obtained by dessication of solutions have been examined by Thermal Gravimetric Analysis (TGA). Powders of VOx /TiO2 were also prepared to be compared with coated plates.

2. Experimental procedure Aluminum foils (1 mm thickness) were anodized in sulphuric acid medium (400 g l
1) for 4 h under direct current (
5bTb
1 8C). Each foil was cut in small pieces (2 cm5 cm1 mm), which were sonicated in acetone to eliminate lubricants traces (20 min), rinsed in water (20 min), and dried at room temperature (3 h). The alumina layers formed on each side of the aluminum foil were 65 Am thick as seen by SEM (Fig. 1). Once alumina was grown on Al plates, a three-step preparation was used. First, a TiO2 monolayer was grafted on alumina/Al in order to ensure the anchoring of a TiO2 (anatase) film, which was then grown in the second step. Finally, VOx species were deposited by grafting on TiO2/Al2O3/Al. 2.1. Coating of plates 2.1.1. First step: grafting of TiO2 monolayer on alumina Different X amounts (0bXb10 g) of titanium tetraisopropoxide Ti(OiPr)4 (Avocado) were solubilized in dry

Fig. 1. SEM micrograph: thickness of the alumina layer formed by anodization.

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propyl-alcohol (Fluka) at room temperature, so as to obtain precisely 100 g of solution. The Al2O3/Al plates were immersed under stirring for 1 h and withdrawn using a home-made apparatus at 6 mm s
1. Plates were then heated in a furnace at 30 8C h
1 up to 500 8C in air flow (5 h). The monolayer of TiO2 grafted on alumina on aluminium plate is further noted TiO2(ML)/Al2O3/Al. 2.1.2. Second step: growing of porous TiO2 on TiO2 (ML)/Al2O3/Al TiO2 films were prepared using the sol–gel method. Various amounts of polyethyleneglycol (Sigma-Aldrich) with an average molecular weight of 2000 g mol
1 (noted PEG), and used as a template, were added in the solution in order to enhance the surface area and the porous volume of the film. The precursor solutions were prepared according to Yu et al. [32]: 17.2 ml of tetrabuthylorthotitanate Ti(OBu)4 97% and 4.8 ml of diethanolamine 99% (both Sigma-Aldrich) were dissolved in dry ethanol (67.28 ml) (Fluka). The solution was stirred vigorously at room temperature for 2 h. About 0 to 1.5 g of PEG, 0.9 ml of water and 10 ml of ethanol (Ti(OBu)4:C2H5OH:H2O:NH (C2H4OH)2=1:25.5:1:1, molar ratio) were added dropwise to the solution under stirring. The resultant alkoxide sols were left standing at room temperature for two hours for the hydrolysis reaction to proceed. TiO2 films were prepared by dipping (20 s) and withdrawing the TiO2 (ML)/Al2O3/Al plates at 6 mm s
1. The resulting plates noted TiO2/Al2O3/Al were calcined in a furnace (30 8C h
1) at 500 8C in air flow for 5 h. To determine this temperature, the powder obtained by drying the corresponding solutions at 60 8C overnight (vide infra) was analysed by TGA (TA Instrument, SDT 2960) (air flow, 10 and 4 8C min
1). In order to increase the thickness and porosity of TiO2 film, the steps of immersion and calcination were repeated 10 times. Samples are noted xPnC, where x is the amount of PEG for 100 g of solution (x: wt.%), and n=10 stands for the number of cycles C (or layers). For example, 0P10C sample is prepared without PEG (0P), and 10 layers (10C) are deposited. 2.1.3. Third step: grafting of VOx monolayer on TiO2/Al2O3/Al Vanadium(V)–oxytripropoxide VO(OPr)3 98% (SigmaAldrich) was used as precursor. Solutions containing different amounts D (wt.% in dry ethanol) of precursor were prepared. TiO2/Al2O3/Al plates were dipped under stirring during 1 h, and then withdrawn (6 mm s
1) from the solution. The plates were then heated (30 8C h
1) at 450 8C for 4 h in air flow. 2.2. Physicochemical analyses The specific surface area and porosity of the film on plates at various stages of coating were determined from the nitrogen adsorption and desorption isotherms to which the

BET and BJH methods were applied. Because of the large weight and size of the metal plate as compared to that of the coating, these isotherms were obtained using a thermobalance (Sartorius, model S3D-V), the reference being a non anodized aluminum plate. For the same reason, it is more appropriate to consider the developed surface area instead of the specific surface area. The partial pressure of nitrogen varied from 104 to 105 Pa at 77 K. All samples were first degassed at 150 8C for 4 h in vacuum. Laser Raman spectra were recorded on a LabRAM Infinity spectrometer (Jobin Yvon) equipped with a liquid nitrogen detector and a frequency-doubled Nd:YAG laser supplying the excitation line at 532 nm. The power on the sample was below 5 mW. The spectrometer was calibrated daily using the silicon line at 521 cm
1. After grafting, the TiO2 and VOx layers were analyzed by XPS using Leybold Heraeus spectrometer. The residual pressure in the ultra-high vacuum chamber was about 10
8 Pa. Al Ka and Mg Ka X-ray sources were used to study TiO2(ML)/Al2O3/Al and VOx /TiO2/Al2O3/Al plates, respectively. The spectra were referenced to Al 2p photopeak (from Al2O3) with binding energy (BE)=74.6 eV for TiO2(ML)/Al2O3/Al samples, and to O 1s photopeak (from TiO2) with BE=530 eV for V2O5/TiO2/Al2O3/Al samples. Surface images were obtained by means of Hitachi 4100 S scanning electron microscope equipped with a Field Emission Gun, with numerical image acquisition.

3. Results and discussion 3.1. TiO2(ML)/Al2O3/Al The powder obtained by drying Ti(OiPr)4 in air at 60 8C was analyzed by TGA to determine the best conditions of calcination of plates. A broad endothermic peak close to 105 8C and a small exothermic peak at 242 8C (Fig. 2) are assigned to the desorption of water and alcohol, and to the decomposition of organic substances contained in the gel, respectively. At 399 8C the sharp exothermic peak (no weight loss) is assigned to the crystallization of TiO2 in the anatase form, as confirmed by LRS carried out on samples after TGA. The end of crystallization taking place at 450 8C, we concluded that a temperature of 500 8C was high enough to ensure the formation of crystalline anatase. When the Al2O3/Al plates are immersed in the solution of Ti(OiPr)4, the following equilibrium proceeds with the surface hydroxyl groups of alumina [33]: surface
OHþTiðOiPrÞ4 X surface
O
TiðOiPrÞ3 þ iPrOH Due to the overwhelming contribution of the metal plate as compared to that of the alumina layer, the number of hydroxyl groups on alumina is difficult to determine by chemical analysis, and therefore the initial amount of Ti(OiPr)4 cannot be known exactly. For this reason, we

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Fig. 2. Thermogravimetric analysis (TGA) of the calcination of Ti(OiPr)4 powder.

have prepared solutions of various concentrations X (wt.%) and determined by XPS the ratio, noted Ti/Al, between the intensity of Ti 2p3/2 and of Al 2p photopeaks of coated plates. When Ti/Al plotted against X reaches a plateau, all hydroxyl groups of Al2O3 have reacted and the theoretical monolayer is supposed to be reached. Table 1 presents the binding energy (BE) and Full Width at Half Maximum (FWHM) of O 1s and Ti 2p3/2 photopeaks in TiO2 (ML)/ Al2O3/Al samples. Two ranges of concentrations are observed, Xb1.5 and 3VXV10. When X=0.25 to 1.5, the binding energy of O 1s is equal to 531.5 eV and FWHM are typical of pure g-Al2O3 [35]. With further increase in Ti(OiPr)4 (X=3–10), the O 1s peak broadens and shifts towards lower BE by c1.5 eV (Fig. 3), down to 529.9 eV which is typical of TiO2-anatase. Simultaneously, Ti/Al increases sharply from 26.5 to 99.0, the latter value corresponding to the precipitation of titania on itself (Table 1). The beginning of the plateau is observed at X=0.8 (Fig. 4). As the O 1s BE in alumina should not be affected by the saturation of hydroxyl groups, we conclude that all OH have reacted for Xc0.8 wt.% of Ti(OiPr)4 in PrOH.

Fig. 3. XPS analysis of TiO2 (ML)/Al2O3/Al plates. Shift of O 1s photopeak (eV) vs. concentration X (Ti(OiPr)4 wt.%).

3.2. Porous TiO2 over TiO2 (ML)/Al2O3/Al TGA of the powders obtained by drying solutions of Ti(OBu)4 with various amounts of PEG (x=0, 0.5, 1.0, 1.5 wt.%) was performed to determine the optimum temperature of calcination of TiO2/Al2O3/Al plates. In the absence of PEG, the two small exothermic peaks at 260 and 330 8C (Fig. 5) are assigned to the decomposition of organic substances contained in the gel. A sharp exothermic peak ca. 515 8C is associated to a loss of weight. LRS performed after TGA shows that TiO2 has crystallized as anatase. Hypothetical transformations have been postulated for each step and their theoretical weight losses (Dm/m, %)

Table 1 XPS analysis of TiO2 (ML)/Al2O3/Al plates varying by concentration X of Ti(OiPr)4 in ethanol (wt.%) Samples

X=0.25–1.5 X=3–10 g-Al2O3 TiO2-anatase

O 1s

[35] [34] [35]

Ti 2p3/2

Ti/Al

BE (eV)

FWHM (eV)

BE (eV)

FWHM (eV)

531.5 529.9 531.5 529.9 530.3

3.2 1.9 3.15 1.6 2.0

459.0 458.6 – 458.7 458.5

– 1.8 – 1.3 1.8

0.16–0.27 26.5–99.0 – –

Binding energy (BE) and FWHM of O 1s and Ti 2p3/2 (F0.2 eV). Ti/Al is the ratio of intensity of Ti 2p3/2 to Al 2p photopeaks.

Fig. 4. Ti/Al intensity ratio as function of the concentration of Ti(OiPr)4 (wt.%) in the XPS analysis of TiO2 (ML)/Al2O3/Al plates.

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Fig. 5. Thermogravimetric analysis of the calcination of Ti(OiPr)4 without PEG (10 8Cmin
1) in air flow.

compared to the experimental ones (Table 2). The numbers being roughly in accordance, the endothermic decomposition of the Ti–N complex (Dm/m=25.3%) proceeds therefore in the same time as the exothermic crystallization of TiO2. An additional endothermic signal ca. 3108 C accounts for decomposition of PEG when present. When the heating rate is decreased to 4 8C/min, the exothermic peak of crystallization is shifted to lower temperature (ca. 470 8C). As the heating rate of the furnace is lower (0.5 8Cmin
1), the temperature of 500 8C was therefore chosen to calcine the plates. Fig. 6 presents the LRS spectra of xP10C plates obtained with various amounts of PEG. Two contributions are superimposed when the TiO2 layer is thin (b1 Am): the alumina layer which is responsible for the fluorescence phenomenon [36], and TiO2 anatase, which is accounted for by a weak band at 150 cm
1. The four bands characteristic

Fig. 6. Raman spectra of TiO2/Al2O3/Al plates: influence of PEG (Samples xP10C with x=0 (a), 0.5 (b), 1.0 (c), 1.5 (d) g of PEG).

of anatase appear at 150, 395, 512, 633 cm
1 [26,37] for samples with 10 layers (Fig. 6) which are thick enough to mask the fluorescence due to Al2O3. Fig. 7 shows the typical nitrogen adsorption–desorption isotherms obtained at 77 K for TiO2/Al2O3/Al plates. The curve exhibits a complex hysteresis due to the presence of two types of pores, named A and B after De Boer [38]. Type A in the range P/P 0=0.8–1.0 is associated with bcylinder shapedQ pores of rather constant cross section, and type B for P/P 0=0.6–0.8 is characteristic of bslit-shapedQ pores. There is no theoretical approach taking into account simultaneously both types of pores, so we have chosen

Table 2 Thermogravimetric analysis of the decomposition of dryed Ti(OiPr)4 powder

Experimental compared to theoretical loss of weight associated to possible reactions.

Fig. 7. Adsorption–desorption isotherms of nitrogen on 0P10C at 77 K.

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the model of cylindrical pores to determine the porous volume distribution. Table 3 gathers values of the developed surface area (m2 per geometrical m2 of plate) which is also defined as the benhancement factorQ by Haas Santo [5]. A perfectly pore-free and flat coating would have a surface enhancement factor of 1 m2/m2, whereas it ranges between 1900 and 2000 m2/m2 for our samples. Fig. 8 represents the porous volume distribution plotted against the radius of the supposedly cylindrical pores. The radii of pores range between 7 and 22 nm for all plates. Macropores and micropores may exist, but the BJH method is not valid to characterize these types. These results show that the effect of PEG on geometric areas is not significant. Small differences in the porous volume distribution are observed but they do not change in a straightforward manner with the amount of PEG. Even though the model we chose is not fully adequate for these complex porous structures, we would have expected to see stronger differences between samples, as those found by Jiaguo et al. for the deposition of TiO2 on slide glass plates [32]. SEM images show (Fig. 9) that, if no pores are observed in plates without PEG, their number and diameter d p (100(PEG=0.5)bd pb500(PEG=1.5) nm) increase with the amount of porogen. Our conclusion is that PEG does not influence the creation of mesopores but that it contributes to the formation of macropores (as seen by SEM), which are not taken into account by BJH method, and which, finally, do not contribute significantly to an increase of the surface area of TiO2. The reason why PEG does not play its role could be related to the metallic plates dipped in the viscous solution which may perturb the deposition of the porogen. In order to check this assumption, TiO2 powders with the same composition (from zero to 1.5 g of PEG) were prepared as above and calcined in the same conditions as plates. Their specific surface area measured by BET increases from 17.5 without PEG to 61.5 m2/g for 1.5 g of PEG (Table 3). Therefore, the expected template effect does exist in the case of powders, but it does not in the case of plates. Probably a phase separation from the solution occurs at the plate/ solution interface, leading to the formation of PEG droplets on the plate surface, which in fine generate the macropores observed by SEM. Yu et al. [39,40] have observed a similar phenomenon during the deposition of titania films with PEG on soda lime glass support. As the viscosity of the gel increased, more phase separation occurred. They found that the diameter of macropores increases with the amount of PEG, and as a consequence the number of pores decreases.

Table 3 Developed surface area of TiO2/Al2O3/Al plates and specific surface area of TiO2-anatase powders according to PEG loading PEG (wt.%)

0

0.5

1

1.5

Plates: Developed surface area (m2/m2F50) Powders: Specific surface area (m2/gF0.5)

1900 17.5

2000 38.0

1900 49.5

1900 61.5

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Fig. 8. Pore volume distribution of xP10C samples varying by the amount of PEG (0bxb1.5 g PEG).

3.3. VOx /TiO2/Al2O3/Al For the same reason as when a TiO2 monolayer was grafted onto Al2O3/Al, the amount of surface hydroxyl groups on TiO2/Al2O3/Al to be grafted by VO(OPr)3 is unknown. XPS was used to determine at which concentration of the VOx precursor the btheoretical monolayerQ is reached after calcination. The binding energies of the V 2p3/2 and Ti 2p3/2 photopeaks were determined using O 1s photopeak as reference (530.0 eV). Table 4 presents the binding energy and FWHM of V 2p3/2 and of Ti 2p3/2, and the ratio (noted V/Ti) between intensity of V 2p3/2 and of Ti 2p3/2 photopeaks obtained for plates prepared with various concentrations D of VO(OiPr)3 in propanol (0.5bDb8 wt.%). The BE and FWHM values of V 2p3/2 and Ti 2p3/2 are in good agreement with the literature data for the V2O5–TiO2 system [41], and are practically unchanged compared to those of pure oxides. For samples with 0.5bDb2.16, V/Ti increases steadily with D, until it reaches a plateau at V/Ti=0.2 as shown on Fig 10. A further increase in VO(OPr)3 concentration (Dz2.4) leads to a sharp increase of V/Ti corresponding to the precipitation of V2O5. The monolayer is therefore obtained at V/Ti=0.2. To compare this value with those obtained on powders, we prepared samples of VOx /TiO2 varying by weight ratio: TiO2 powder (ALFA AESAR, 50 m2/g) was impregnated by various amounts of NH4VO3 in oxalic acid [42,43], and calcined in the same conditions as plates. The theoretical monolayer should be reached for V2O5/ TiO2=3.5 wt.%. By XPS (Table 4), we observe indeed that V/Ti increases linearly up to 0.2, after which value the slope is far smaller for compositions E=V2O5/TiO2 higher

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Fig. 9. SEM micrographs (10 000) of TiO2/Al2O3/Al plates: Influence of PEG on porosity (0 (a), 0.5 (b), 1 (c), 1.5 (d) g of PEG).

than 3.5 wt.%. A similar plateau at V/Ti=0.2 was already obtained by Bond [42–44], while Mendialdua et al. [41] observed it for V/Ti=0.3. Therefore, V/Ti characterizing VOx monolayers when supported by TiO2 is independent from the shaping of the anatase support (powder or plate). This is a very important point as the control of the amount of vanadium oxide deposited on TiO2 is crucial for the catalytic properties of the material. Bond et al. [42,43] have demonstrated that, as the V loading increases, the formation of the monolayer is followed by the building of a disordered VOx phase in the one to four monolayers equivalent range, and then by a paracrystalline V2O5 phase exposing principally faces normal to the basal (010) plane. These blocks which grow into microcrystalline dtowersT cover only a limited part of the surface. Although XPS

measurements do not allow to discriminate between these forms, this model is quite universally adopted [8,10,45,46]. In particular, Grzybowska [46] has proposed a clear picture of what type of VOx is found on the support surface, as schematized on Fig. 11. According to the amount of vanadium expressed in monolayer (ML) units, isolated monomeric vanadium tetrahedra and polymeric vanadates are formed in the 0–0.2 and 0.2–1.0 ML ranges, respectively (Fig. 11). Upon increasing V loading, amorphous polymeric species (1–5 ML) and then V2O5 crystallites (N5 ML) are formed, as checked by many experiments in literature. To conclude, the plateau deter-

Table 4 XPS analysis of VOx /TiO2/Al2O3/Al plates and of VOx /TiO2 powders varying by concentration D of VO(OPr)3 (wt.%) or by E=V2O5/TiO2 ratio (wt.%), respectively Samples

D=0.5–2.2 D=2.4–8 E=0.5–3.5 E=6.25–20 TiO2[34] [39] V2O5[34] [39]

V 2p3/2

Ti 2p3/2

O 1s

V/Ti

BE (eV)

FWHM BE (eV) (eV)

FWHM BE (eV) (eV)

FWHM (eV)

517.0 517.2 517.3 517.4 –

2.0 1.8 1.9 1.4

1.6 1.6 1.2 1.2 1.3 1.5 –

1.8 1.8 1.8 1.8 1.6 1.7 1.5 1.6

517.4 1.5 517.0 1.4

458.8 458.9 458.6 458.6 458.7 458.5 –

530.0 530.0 530.0 530.0 529.9 530.0 530.2 530.0

0.1–0.2 0.3–8.0 0.1–0.2 0.31– 0.62 – –

Binding energy (BE) and FWHM of V2p3/2 and Ti 2p3/2 (F0.2 eV). V/Ti is the ratio of intensity of V2p3/2 to Ti 2p3/2 photopeaks.

Fig. 10. V/Ti intensity ratio as a function of the concentration of VO(OPr)3 (wt.%) in the XPS analysis of VOx /TiO2/Al2O3/Al plates.

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71

Fig. 11. Scheme of the various [VOx ] species displayed on TiO2 anatase support [after Ref. [46]].

mined by XPS corresponds to the formation of the VOx polyvanadates monolayer, followed by building of V2O5 crystals.

4. Conclusion The dip-coating technique using metallic alcoholates is a valuable method for the grafting of VOx /TiO2 on Al2O3/Al plates. It allows a good control of the loading and the resulting films are mechanically stable. However, compared to the well-characterized powders of VOx /TiO2 catalyst, the catalytic metallic plates are not straightforwardly realized nor characterized. The use of LRS, TGA, BET techniques is complicated by the contribution of the metallic plate, while usual X-ray diffraction devices are not suited. The chemistry itself is difficult to transpose from powders to metallic plates. In particular, adding a porogen like PEG to increase the specific surface area of TiO2 is efficient when working on powders, whereas on plates the demixtion of PEG from the solution proceeds at the plate/solution interface. The resulting droplets of PEG on the surface of the plate generate macropores instead of the expected mesopores. Moreover, whereas on powder catalysts the surface Ti/Al or V/Ti compositions are directly determined by the stoechiometry of the impregnation solution, this is not the case when using metallic carriers. This difficulty is overcome by the use of XPS. Finally, our results on the formation of VOx monolayers on anatase are in good agreement with those found in the literature for powders, which demonstrates that the shaping of the anatase support (powder or plate) has not modified the properties of the VOx /TiO2-anatase system. The dipcoating is shown to be a valuable technique to deposit desired amounts of vanadium oxide on titania, and thus to control the species grafted on the support. The study of their thermal and chemical stability, as well as of their catalytic properties in the oxidative dehydrogenation of propane to propene, is in progress.

Acknowledgements O. Gardoll and L. Gengembre are thanked for thermal analyses and XPS experiments, respectively.

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