Effect of doping of TiO2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia–titania catalysts

Applied Catalysis A: General 230 (2002) 1–10

Effect of doping of TiO2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia–titania catalysts B. Grzybowska a,∗ , J. Słoczy´nski a , R. Grabowski a , K. Samson a , I. Gressel a , K. Wcisło a , L. Gengembre b , Y. Barbaux c b

a Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30-239 Kraków, Poland Laboratoire de Catalyse Homogène et Hétérogène, UPRESA CNRS 8010, Université des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq Cedex, France c Faculté des Sciences Jean Perrin, Université d’Artois, SP 18, 62307 Lens Cedex, France

Received 21 June 2001; received in revised form 7 August 2001; accepted 5 November 2001

Abstract Vanadia phase (one monolayer) was deposited on TiO2 anatase doped with Ca2+ , Al3+ , Fe3+ and W6+ ions and the catalysts thus obtained (VMeTi) were characterized by XPS, work function technique, decomposition of isopropanol (a probe reaction for acido–basic properties) and tested in oxidative dehydrogenation of propane. The doping of the TiO2 support modifies physicochemical and catalytic properties of the active vanadia phase with respect to the undoped TiO2 . The specific activity in the propane oxydehydrogenation decreases in the order: VFeTi > VWTi > VTi > VAlTi > VCaTi (3), whereas the selectivity to propene follows the sequence: VWTi < VTi < VFeTi < VAlTi < VCaTi (4). No clear correlation has been observed between the activity of the catalysts and the acido–basic properties. On the other hand, the rate of the isopropanol dehydration (the acidity) decreases in the same order as the selectivity increases (4). The increase in the selectivity with the decrease in the acidity is ascribed to the easier desorption of propene from the less acidic surface, preventing the consecutive total combustion of propene. The selectivity to propene can be also correlated with the work function values which decrease in the series: VWTi > VTi > VFeTi > VAlTi > VCaTi. This implies that the lower is the surface energy barrier for transfer of electrons from the catalyst to the reacting molecules the higher is the selectivity to the partial oxidation product. It is argued that owing to the decrease in this energy barrier the reoxidation step in the catalytic reaction, involving such a transfer: O2 + 4e → 2O2− is fast, thus, preventing the presence of intermediate non-selective electrophilic oxygen species on the surface. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Oxidative dehydrogenation; Propane; Vanadia–titania catalysts

1. Introduction Vanadia–titania system is a well known catalyst of numerous selective oxidation reactions and extensive ∗ Corresponding author. Tel.: +48-12-639-5100; fax: +48-12-425-1923. E-mail address: [email protected] (B. Grzybowska).

studies have been devoted to characterization of its structure, physicochemical properties and their bearing on catalytic performance in the oxidation reactions ([1] and references therein, [2] and papers therein). Relatively few papers have been concerned with the effect of additives on the properties of V2 O5 –TiO2 catalysts ([3–6], [7] and references therein, [8–15])

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 9 5 1 - 6

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most attention being paid to the alkali metals additives and their effect on acido–basic properties. The additives were usually introduced together with the precursor of the vanadia phase on the surface of the titania support in the impregnation step of the catalyst preparation. A small difference in physicochemical and catalytic properties was, however, observed for the samples in which the order of the introduction of the potassium additive (vanadia phase on K-doped TiO2 , or K added to performed vanadia/titania) was different [15]. Some studies were also concerned with the V2 O5 –WO3 /TiO2 catalysts for De-NOx processes, which were prepared by deposition of vanadia on WO3 /TiO2 samples [16]. According to Wachs and coworkers ([4], [7] and references therein) some of the additives (e.g. WO3 , SiO2 , and Nb2 O5 ) are anchored to the surface of the titania with lateral, indirect interactions with the vanadia surface species, whereas others (e.g. K or P) interact directly with the vanadia phase. There has been no systematic study on the effect of doping of the titania support on the properties of an oxide phase deposited on the surface of the doped support. Several works report, on the other hand, such the effect in the case of metals (e.g. Pt or Rh) supported on titania doped with altervalent ions (e.g. W6+ , Zn2+ , Al3+ ) [17–20]. Changes of catalytic properties in hydrogenation of CO [17–19], oxidation of CO [19] or NO decomposition [20], observed for metal catalysts on the doped supports, were interpreted in terms of electronic interactions between the active metal phase and the semiconducting support, the electrical properties of which can be modified by dopants. In the present work, the catalysts containing vanadia supported on TiO2 -anatase doped with ions of valence higher (W6+ ), or lower (Al3+ , Fe3+ , Ca2+ ) than Ti4+ , have been prepared and tested in oxidative dehydrogenation of propane, a reaction of considerable interest in recent years as a source of cheap propene [21–23]. Previous studies have shown that vanadia–titania system is a promising catalyst in this reaction [11–13]. The supports and catalysts have been characterized by XPS technique, and by probe reaction of acido–basic properties (decomposition of isopropanol). Their electronic properties have been studied by the measurements of work function which provides information about the top uppermost layer of the surface. Studies of electrical conductivity of the system will be a subject of a separate paper [24].

2. Experimental 2.1. Preparation of the supports and catalysts TiO2 -anatase (Tioxide batch NP 93/204, specific surface area 48 m2 ) was used for preparation of the doped supports and the catalysts. The impurities detected by the spectral analysis of the support as received were as follows (in wt.%): Mg (0.01), Al (0.01), P (<0.1), Na (<0.1), Si (ca. 0.03). The XPS analysis of the support showed small amounts of S as the only impurity on the support surface. This impurity disappeared, however, after calcination at 750 ◦ C. The doped supports were obtained by impregnation of the support by solutions of Ca, Al, and Fe2+ nitrates and ammonium tungstate, evaporation and calcination for 5 h at 750 ◦ C with the exception of the Fe-containing sample, for which the anatase–rutile transformation was observed at this temperature. The latter was calcined then at 650 ◦ C at which no such transformation occurred. A sample of the original support without doping was also calcined at 750 ◦ C. The content of the dopants corresponded to 5 mol% of Ca, Al and W and 1.4 mol% of Fe (reported as the solubility limit of Fe in TiO2 ). Vanadia was introduced by impregnation of the supports thus obtained with a solution of ammonium metavanadate, followed by evaporation, drying for 5 h at 120 ◦ C and calcination for 5 h at 500 ◦ C under a stream of air. The vanadia content corresponded to one theoretical monolayer of V2 O5 calculated from the crystallographic data as equivalent to 10 vanadium atoms per nm2 . The supports and catalysts are designated further in the text by symbols XTi and VXTi where X is a dopant element. The specific surface area of the supports and catalysts was determined with the BET method using argon as a sorbate. 2.2. XPS The XPS spectra of the samples were recorded with a Leybold Heraeus LHS 10 spectrometer using an Al K␣ X-ray source with incitent radiation at 1486.6 eV. The base pressure was 10−8 Torr. The surface composition was calculated from the intensity ratios IA /IB of the elements using the formula
1.77 IA σA EAkin nA = (1) kin IB σ B EB nB

B. Grzybowska et al. / Applied Catalysis A: General 230 (2002) 1–10

where σ are the cross-sections taken from [25] and nA and nB the numbers of atoms on the surface. 2.3. XRD The XRD diffractograms were recorded with a Philips PW 1710 diffractometer with Cu K␣ radiation. The lattice parameters were calculated using LATCON program. 2.4. Catalytic activity measurements The activity of the catalysts in oxidative dehydrogenation (ODH) of propane was measured in a fixed bed flow apparatus at 280 ◦ C. A stainless steel reactor (120 mm long, i.d. 13 mm) was coupled directly to a series of gas chromatographs. Propene and carbon monoxide were found to be main reaction products, small amounts of carbon dioxide were also observed. The amounts of the degradation, C2 products and of oxygenates (acrolein and acrylic acids) were <1% of the total amount of products. The reaction mixture contained 7.1 vol.% of propane in air. One to two gram samples of a catalyst of grain size 0.63–1 mm diluted with quartz beads were used. To obtain comparable conversions (5 and 10%) for different catalysts the contact time was varied between 0.2 and 1 s by changing the total flow rate of the reaction mixture. It has been checked in the preliminary measurements in which the mass and the grain size of a catalyst sample were varied, that under these conditions the transport phenomena do not limit the reaction rate. The carbon balance for conversions higher than about 10% was better than 97 ± 2%. At lower conversions the balance was poorer and, hence, the selectivities to different products were calculated from the formula: Si = ci /Σci where ci are concentrations of products i. The total conversion Xp was calculated as: Xp =

cp0 − cp cp0

3

2.5. Isopropanol decomposition Decomposition of isopropanol, (isoPrOH) to propene and acetone ether was studied at 170 ◦ C with the pulse method, using dried helium as a carrier gas. A total of 0.1 g of the sample and 2 ␮l pulses of isoPrOH were used, the total flow rate of helium being 30 ml/min. Analysis of products was performed with gas chromatography with FID detection. The isoPrOH pulses were injected successively till constant values of conversion and amounts of products were attained (usually after 3–5 pulses). The values of amounts of products per pulse reported further in the text are the mean of values obtained in three successive pulses after the stationary state of activity was obtained. Before the experiments the samples were standardized in a stream of helium for 2 h at 200 ◦ C. 2.6. Work function measurements Work function measurements were performed with the vibrating condenser method in an apparatus described in detail in [26]. The sample was placed from the acetone suspension on a gold foil enveloping one of the condenser plates. A small electric resistance heater located on the outer face of this plate made it possible to heat the sample up to ∼500 ◦ C. The sample temperature was measured by a thermocouple placed at the plate. The vibrating, reference electrode was made of graphite plate, stable under measurement conditions [26]. The values of Φ, reported further in the text (expressed in mV) are the Volta potential difference between two plates of a condenser formed by a sample deposited on a golden foil and a reference electrode. They correspond to the difference in the work function: Ψ = eΦ, where Ψ is the work function and e an electron charge. In the adopted set-up the increase in Φ indicates that the surfaces becomes more negatively charged. The measurements were performed in the temperature range 100–500 ◦ C under a flow of 20% O2 in argon.

(2)

where cp0 and cp are the concentrations of propane at the inlet and outlet of the reactor, respectively. The doped TiO2 supports were not active in the studied conditions, the total propane conversion not exceeding 1% at 280 ◦ C.

3. Results and discussion 3.1. Basic characteristics of the catalysts Table 1 gives the values of the specific surface area of the supports and the catalysts and the XRD data for

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Table 1 Basic characteristics of the samples Sample

Specific surface area (m2 /g)

a =b Ti VTi WTi VWTi FeTi VFeTi AlTi VAlTi CaTi VCaTi a

22.5 17.7 38.6 29.9 38.9 38.5 31.4 24.6 31.3 35.3

Ionic radiusa (Å)

Lattice parameters (Å)

3.7833 – 3.7831 – 3.7841 – 3.7823 – 3.7822 –

c ± 0.0006 ± 0.0008 ± 0.0014 ± 0.0009 ± 0.0018

9.5115 – 9.5107 – 9.5113 – 9.5091 – 9.4953 –

± 0.0018

0.605 (Ti4+ )

± 0.0024

0.600 (W6+ )

± 0.0042

0.645 (Fe3+ )

± 0.0026

0.535 (Al3+ )

± 0.0054

1.000 (Ca2+ )

From [40].

the supports. The specific surface area, Ssp of the original titania decreases after calcination at 750 ◦ C. The changes in Ssp are smaller for the doped samples as compared with that without doping, which indicates that the additives hinder the sintering of TiO2 . The XRD analysis has shown the presence of anatase TiO2 as the only crystalline phase both in the supports and the catalysts. The absence of the pattern of vanadia could suggest that this oxide is present in the form of dispersed VOx species, still the total content of vanadia is rather small (2–3 wt.%) and may be below the limits of detection with the XRD technique. No lines corresponding to the oxides of the dopant elements have been observed either. The lattice parameters of pure and doped TiO2 used in the studies are listed in Table 1. Taking into account the atomic radii of the dopants with respect to that of the Ti4+ host lattice ion, one would expect a small contraction of the lattice in the case of the substitution with Al3+ ion and an expansion in the case of Fe3+ ion. The radius of Ca2+ appears too big to allow this ion to enter the TiO2 lattice. Small contraction is indeed observed in the case of the Al-doped support, and small expansion in the case of the Fe-doped titania. The latter can be ascribed to the presence of Fe2+ ions (atomic radius 0.78 Å), indicated by the XPS data (see the next paragraph). The decrease in the lattice parameters found for the Ca-doped support cannot be explained in view of the big atomic radius of calcium ion. Still the error in the estimation of the lattice parameters in the doped samples is rather high and does not permit to identify with certainty the manner of incorporation of the dopants.

3.2. XPS measurements Table 2 lists the binding energies (BE) of different elements in the studied samples of the supports and catalysts, the Ti (2p3/2 ) signal with E b = 459 eV being taken as a reference. The values of BE for the O (1s) level are 530.3 ± 0.3 eV, those of V (2p3/2 ) are 517.1 ± 0.3 eV (the value typical of the V5+ ions) for the catalysts on the doped supports. Lower (by ca. 0.6 eV) value observed for BE of vanadium in the catalyst on undoped TiO2 indicates higher electron density around V ion in this sample, i.e. partial reduction of V5+ ions. Partial reduction of the vanadia phase present in a monolayer structure in V2 O5 –TiO2 catalysts has been previously evidenced by ESR and chemical analysis [27] and ascribed to the interaction of vanadia species with the TiO2 surface. The presence of dopants on the titania surface appears then to hinder the reduction of vanadium, suggesting that the interaction of the vanadia phase is weaker. The values of the BE of the dopant elements indicate the presence of W6+ , Ca2+ , Al3+ ions in both the supports and respective catalysts. Iron is present in the form of Fe2+ ions on the Fe–TiO2 support surface and of Fe3+ ions in the VOx /Fe–TiO2 catalysts. The surface atomic ratio V/Ti does not vary significantly in the studied series of the catalysts, suggesting that the presence of the dopants does not affect significantly the dispersion of the vanadia phase. The atomic ratios dopants/Ti are similar for the supports and for the catalysts which suggests that the vanadia

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Table 2 XPS data for vanadia supported on titania doped with altervalent ions Sample

Ti VTi WTi VWTi FeTi VFeTi AlTi VAlTi CaTi VCaTi

BE and FWHM (in brackets) (eV)

Surface composition

Ti (2p3/2 )

O (1s)

V (2p3/2 )

Dopants, X

nv /(nTi + nX )

nX /(nTi + nX )

459 459 459 459 459 459 459 459 459 459

530.2 530.3 530.1 530.6 530.0 530.2 530.4 530.2 530.5 530.2

– 516.4 – 517.0 – 516.9 – 517.4 – 516.8

– – 247.6 247.4 709.4 710.6 119.5 119.2 247.6 247.6

– 0.11 – 0.12 – 0.10 – 0.13 – 0.14

– – 0.056 0.060 0.052 0.04 0.12 0.15 0.11 0.09

(1.6) (1.8) (1.6) (1.9) (1.8) (1.6) (1.6) (1.8) (1.7) (1.8)

(1.8) (1.9) (1.8) (2.4) (2.0) (1.8) (1.8) (2.0) (1.8) (1.8)

(2.9) (3.3) (2.3) (3.2) (2.7)

phase is located beside the atoms of the dopants on the support surface. 3.3. Catalytic activity Fig. 1 shows changes of conversion of propane and selectivity to different products with the reaction temperature for a VTi catalyst without additives. Similar course of changes has been observed for all the studied catalysts, and is typical for the ODH of propane on vanadia-based catalyst [28–30]. The in-

(4d5/2 ) [W6+ ] (4d5/2 ) [W6+ ] (2p3/2 ) [Fe2+ ] (2p3/2 ) [Fe3+ ] (2s) [Al3+ ] (2s) [Al3+ ] (2p3/2 ) [Ca2+ ] (2p3/2 ) [Ca2+ ]

crease of the conversion with the temperature is accompanied by the decrease in the selectivity to propene and the increase in the selectivity to CO, which indicates the consecutive route of formation of carbon monoxide from propene, formed in the first step of the reaction. The selectivity to CO2 is practically constant with the temperature (with the increasing conversion), which implies that carbon dioxide is formed mainly by a parallel route, directly from propane. In Table 3, activities and selectivities to propene at the same values of total conversion, 5 and 10% (S5 and S10 ) at 280 ◦ C are compared for all the catalysts under study. The values of selectivities for the VTi catalyst are in the range of those reported previously for vanadia deposited on anatase–TiO2 (15–30% at 10% conversion, depending on specific surface area and provenance of TiO2 ) [11–13,29,30]. Highest selectivity obtained in the present work (45% at 10% conversion), observed for the system containing alkaline earth additive (VCaTi catalyst) can be compared with Table 3 Catalytic data at 280 ◦ C for vanadia supported on titania doped with altervalent ions Catalyst

Fig. 1. Propane conversion (䊊), and selectivity to propene (䊏), CO (䉱) and CO2 (䉬) in oxidative dehydrogenation of propane on VTi catalyst as a function of the reaction temperature.

VTi VWTi VFeTi VAlTi VCaTi

Vsp (×106 ) (mol C3 H8 min–1 m2 )

1.3 1.6 5.6 0.6 0.4

Selectivity to propene at conversion 5%

10%

30 23 – 52 55

18 11 20 35 45

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Table 4 Decomposition of isopropanol on vanadia supported on titania doped with altervalent ions Sample

C3 H6 (×106 ) (mol/m2 s)

C3 H6 O (×106 ) (mol/m2 s)

C3 H6 O/C3 H6

Ti VTi WTi VWTi FeTi VFeTi AlTi VAlTi CaTi VCaTi

0.05 0.24 1.10 0.90 0.01 0.20, 0.70a 0.02 0.09 0.03 0.02

0.17 1.70 0.50 0.45 0.22 0.97, 0.41a 0.13 0.31 0.12 0.46

3.40 7.10 0.45 0.50 2.20 4.80, 0.87a 6.50 3.40 4.00 23.00

a

First pulse.

the selectivity to propene found for alkali metal doped vanadia–titania (45–58% at 10% conversion) [11–13]. The data show the distinct effect of the doping of the TiO2 support with altervalent ions on the catalytic performance of the deposited vanadia phase. The specific activity: VFeTi > VWTi > VTi > VAlTi > VCaTi

(3)

phase. The dehydrogenating properties of the vanadia phase supported on doped TiO2 are suppressed in comparison with that on the undoped support. The rate of the propene formation, for the catalysts (a measure of the acidity) follows the sequence: VWTi > VTi > VFeTi > VAlTi > VCaTi

(5)

Whereas that of the acetone formation the sequence:

Whereas the selectivity to propene follows the sequence:

VTi > VFeTi > VAlTi VWTi = VCaTi

VWTi < VTi < VFeTi < VAlTi < VCaTi

The acetone/propene ratio, which according to Ai [31] can be taken as a measure of the basicity, is:

(4)

(6)

3.4. Decomposition of isopropanol

VCaTi > VTi > VFeTi > VAlTi > VWTi

Table 4 presents the data of the isopropanol decomposition for the doped supports and the catalysts. The doping of the support with altervalent ions modifies their properties in this reaction. Generally, the changes are small with the exception of W-doped TiO2 which presents much higher activity in both dehydration and dehydrogenation of isopropanol as compared with the undoped and doped with other ions titania. It can be mentioned that tungsta was found to impart strong Lewis and Brönsted acidity to titania when it was deposited on TiO2 [16]. The catalysts with the deposited vanadia phase show on the whole higher activities in the reaction than the supports, with the exception of VWTi sample, for which the support (WTi) itself was relatively very active. The observed changes in the rate of formation of the isopropanol decomposition products can be then ascribed in part to the modification of the acido–basic properties of the deposited vanadia

In the above sequences, the stationary state data for the VFeTi catalyst were taken. Higher rate of the isopropanol dehydration to propene and lower of dehydrogenation observed for this catalyst in the first pulse as compared with those in the stationary state may be due to partial reduction of Fe3+ to Fe2+ ions by isopropanol: Fe2+ could be expect to have lower acidity and more pronounced dehydrogenating properties than Fe3+ . In view of the fact that propane/air mixture in the ODH reaction may have some reducing properties, adoption of the isopropanol decomposition data for the stationary (reduced state) in sequences (5)–(7) seemed more appropriate. The activity of the W-doped support and the uncertainty about the state of Fe in the working catalyst render the more advanced discussion about the effect of the modifications of the acido–basic properties of vanadia phase on catalytic properties difficult. It can only be observed that no clear correlation has been found between the activity of the catalysts

(7)

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in the ODH of propane and the rate of decomposition of isopropanol to propene and acetone, nor to the basicity. On the other hand, one can remark that the sequence of the increasing selectivity to propene in the ODH reaction (4) follows that of the decreasing rate of the propene formation in the isopropanol decomposition (the acidity) (sequence (5)). The increase in the selectivity to the olefin with the decrease in the acidity of the catalysts was reported for several cases of the ODH reactions, e.g. for the ODH of propane on alkali metal doped vanadia–titania catalysts [11–13]. It can be ascribed to the weaker interaction of a basic propene molecule with less acidic surface: propene is then easily desorbed from the surface before undergoing consecutive oxidation to carbon oxides. 3.5. Work function Figs. 2 and 3 present the changes in the contact potential difference Φ, (related to work function as shown in p.7) with temperature in the range 100– 500 ◦ C for the supports and the catalysts under study. The values of Φ of both the supports and the catalysts with respect to the undoped samples are higher for the

7

W6+ doped samples and lower for samples doped with ions of lower valency than Ti4+ . Over the whole range of temperature studied the values of Φ of both the supports and the catalysts decrease in the sequence: VWTi > VTi > VFeTi > VAlTi VCaTi

(8)

The values increase significantly with temperature indicating an increase in the negative charge on the surface. Such an increase can be ascribed to transformation of the chemisorbed oxygen species to more − negatively charged forms, e.g. O2 +e− →O− 2 , or O + − 2− e → O [32], it can be also due to an increase in the amount of chemisorbed oxygen with temperature. Deposition of vanadia phase on the supports leads to the increase in the Φ values, most significant for the CaTi support. The changes of the work function values for the samples doped with altervalent ions may be due to the shift of the Fermi energy level EF in the band structure of TiO2 (an n-semiconducting oxide) resulting from the introduction of electronic or atomic defects into the parent lattice, and to the changes in the dipole layer in the outermost layer of the solid. On an oxide surface the polarized Me–Ox groups with oxygen ions sticking out of the surface give rise to the surface

Fig. 2. Contact potential difference of titania undoped (䊊) and doped with W (䊉), Fe (䉱), Al (䉬) and Ca (䊏) as a function of temperature: measurements in air.

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Fig. 3. Contact potential difference of vanadia catalysts on undoped and doped titania: VTi: (䊊) VWTi: (䊉) VFeTi: (䉱) VAlTi: (䉬) VCaTi: (䊏) as a function of temperature: measurements in air.

negative charge, whereas those with the exposed cations to the positive charge. The chemisorbed oxygen ions increase the negative charge on the surface, leading to the increase in the work function. The structure of defects in the studied samples is not known and would require special studies. In the simplest case, assuming simple substitution of Ti4+ with the altervalent ions, one would expect the increase in the electron concentration (appearance of Ti3+ ions) and upward shift of EF when W6+ is introduced, and decrease in the electron concentration when ions of lower valency than Ti4+ are present. One can expect that in the first case the adsorption of oxygen with formation of ionosorbed oxygen species will be favored giving rise to the higher surface potential values in the oxygen atmosphere, whereas the lower concentration of electrons would decrease the coverage of the surface with ionosorbed species, and thus, lower the measured work function. Other type of defect structure may be also envisaged, e.g. for the Al-doped TiO2 the presence of interstitial Al3+ ions beside substitutional ions was reported [33]. One can also speculate that in the case of Ca-doping, a Ca cation of large radius, which cannot enter the

TiO2 lattice remains on the surface blocking some oxygen ions and, hence, changing the structure of the dipole surface layer (decreasing the negative charge on the surface, and hence, the surface potential). It can be recalled that the considerable decrease in the work function values was observed previously for the K-doped TiO2 or alkali doped alkali metals in the outermost layer of the samples [11,15,30,34]. Irrespectively of the mechanism, which is responsible for the modification in the work function in oxygen atmosphere, it can be observed that the sequence of the work function values (8) corresponds to the sequence (4) of the increasing selectivity to propene in the ODH of propane. This implies that, the lower is the energy barrier for the transfer of electrons from a catalyst to the molecules reacting on the surface the higher is the selectivity to the partial oxidation product. In the oxidation processes, the transfer of electrons from the catalyst to the reacting molecules is involved in the reoxidation step described by the overall reaction: O2 + 4e → 2O2− , which proceeds most probably through successive formation of O2 − , and atomic O− species ([35], [36,37] and references therein). The electrons necessary for this step coming from a catalyst have to

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surmount an energy barrier equal to the surface potential. The exponential dependence of the rate of catalytic reactions on the catalyst work function has been in fact found by Vayenas and coworkers [38,39]. The lower energy barrier for the electron transfer would facilitate the above reaction and decrease the possibility of the presence on the catalyst surface of molecular or atomic electrophilic oxygen forms, which take part in non-selective total oxidation of hydrocarbons. On the other hand, the lower is the Φ the weaker will be the chemisorptive bond of electron donor-propene, which would also favor the higher selectivity to propene.

1. Physicochemical and catalytic properties of vanadia phase (one monolayer) deposited on TiO2 anatase doped with Ca2+ , Al3+ , Fe3+ and W6+ ions are modified as compared to those on non-doped titania. 2. The presence of dopants slows down the sintering of TiO2 , the specific surface area of the doped TiO2 support being higher than that of the undoped sample heated at the same temperature. 3. The dispersion of vanadia phase is not affected significantly by the presence of the dopants as suggested by the values of V/Ti ratio registered by the XPS technique. The higher BE’s of vanadium for the catalysts on doped support indicate, on the other hand, that the dopants hinder the partial reduction of vanadium, most probably by weakening the vanadia phase–support interaction. 4. The specific activity of vanadia–doped-titania catalysts in oxidative dehydrogenation of propane follows the sequence: (3)

Whereas the selectivity to propene increases in the series: VWTi < VTi < VFeTi < VAlTi < VCaTi

(5)

The increase in the selectivity to propene with the decrease in the acidity could be due to the easier desorption of propene on less acidic surface, preventing consecutive oxidation of the olefin to carbon oxides. 5. There is no clear correlation between the specific activity of the catalysts and the rate of either dehydration or dehydrogenation of isopropanol. 6. The work function of both the support and the catalysts is modified by the doping of titania. The sequence of the decreasing work function values: VWTi > VTi > VFeTi > VAlTi VCaTi

4. Conclusions

VFeTi > VWTi > VTi > VAlTi > VCaTi

VWTi > VTi > VFeTi > VAlTi > VCaTi

9

(4)

There is a correlation between the selectivity to propene and the acidity of the catalysts measured by the rate of the isopropanol dehydration to propene, the acidity decreasing in the sequence:

corresponds to that of the increasing selectivity to propene (4). The increase in the selectivity with the decrease in the work function (i.e. with the decrease in the energy barrier for the transfer of electrons from the catalyst to the reacting molecules) can be ascribed to the faster reoxidation of the catalyst O2 + 4e− → 2O2− , which renders the presence of intermediate nonselective electrophilic O2 − and O− species on the surface less probable. References ´ [1] B. Grzybowska-Swierkosz, Appl. Catal. A: Gen. 157 (1997) 263. [2] J.C. Vedrine (Ed.), Catalysis Today, Eurocat Oxide, Vol. 20, No. 1. [3] M.G. Nobbenhuis, P. Hug, T. Mallat, A. Baiker, Appl. Catal. A 108 (1994) 241. [4] G. Deo, I.E. Wachs, J. Catal. 146 (1994) 335. [5] G.K. Boreskov, A.A. Ivanov, O.M. Ilyinich, V.G. Ponomareva, React. Kinet. Catal. Lett. 3 (1975) 1. [6] J. Zhu, S.L.T. Andersson, J. Chem. Soc., Faraday Trans. I 85 (1989) 3629. [7] G. Deo, I.E. Wachs, J. Haber, Crit. Rev. Surf. Chem. 4 (1994) 141. [8] C. Martin, V. Rives, J. Catal. 114 (1988) 473. [9] C. Martin, V. Rives, J. Mol. Catal. 48 (1988) 381. [10] L. Lietti, P. Forzatti, Appl. Catal. B: Environ. 3 (1993) 13. [11] B. Grzybowska, P. Mekss, R. Grabowski, K. Wcisło, Y. Barbaux, L. Gengembre, Stud. Surf. Sci. Catal. 82 (1994) 151. [12] R. Grabowski, B. Grzybowska, K. Wcisło, Polish J. Chem. 68 (1994) 1803. [13] R. Grabowski, B. Grzybowska, K. Samson, J. Słoczynski, J. Stoch, K. Wcisło, Appl. Catal. A: Gen. 125 (1995) 129.

10

B. Grzybowska et al. / Applied Catalysis A: General 230 (2002) 1–10

[14] D.A. Bulushev, L. Kiwi-Minsker, V.I. Zaikovskii, O.B. Lapina, A.A. Ivanov, S.I. Reshetnikov, A. Renken, Appl. Catal. A: Gen. 202 (2000) 243. [15] D. Courcot, A. Ponchel, B. Grzybowska, Y. Barbaux, M. Rigole, M. Guelton, J.P. Bonnelle, Catal. Today 33 (1997) 109. [16] L.J. Alemany, L. Lietti, N. Ferlazzo, P. Forzatti, G. Busca, E. Giamello, F. Bregani, J. Catal. 155 (1995) 117. [17] F. Solymosi, I. Tombácz, J. Koszta, J. Catal. 95 (1985) 578. [18] T. Ioannides, X.E. Verykios, M. Tsapatsis, C. Economou, J. Catal. 145 (1994) 491. [19] V.G. Papadakis, C.A. Pliangos, I.V. Yentekakis, X.E. Verykios, C.G. Vayenas, Catal. Today 29 (1996) 71. [20] T. Chafik, A.M. Efstathiou, X.E. Verykios, J. Phys. Chem. B 101 (1997) 7968. [21] H.H. Kung, Adv. Catal. 40 (1994) 1. [22] S. Albonetti, F. Cavani, F. Trifirò, Catal. Rev. Sci. Eng. 38 (4) (1996) 413. [23] F. Cavani, F. Trifirò, Catal. Today 36 (1997) 431. [24] M. Caldararu, in preparation. [25] J.H. Scofield, J. Electron. Spectrosc. 8 (1976) 129. [26] Y. Barbaux, J.P. Bonelle, J.P. Beaufils, J. Chim. Phys. 73 (1976) 25.

[27] M. Rusiecka, B. Grzybowska, M. G˛asior, Appl. Catal. 10 (1984) 101. [28] J. Słoczy´nski, R. Grabowski, K. Wcisło, B. Grzybowska´ Swierkosz, Polish J. Chem. 71 (1997) 1585. [29] N. Boisdron, A. Monnier, L. Jalowiecki-Duhamel, Y. Barbaux, J. Chem. Soc., Faraday Trans. 91 (1995) 2899. [30] D. Courcot, L. Gengembre, M. Guelton, Y. Barbaux, B. Grzybowska, J. Chem. Soc., Faraday Trans. 90 (1994) 895. [31] M. Ai, Bull. Chem. Soc. Jpn. 49 (1976) 1328. [32] Y. Barbaux, J.P. Bonelle, J.P. Beaufils, J. Chem. Res. (S)48 (1979) (M)0556. [33] A. Boronocolos, J.C. Vickerman, J. Catal. 100 (1986) 59. [34] R. Grabowski, B. Grzybowska, A. Kozłowska, J. Słoczy´nski, K. Wcisło, Topics Catal. 3 (1996) 277. [35] A. Biela´nski, J. Haber, Oxygen in Catalysis, Marcel Dekker, New York, 1991. [36] M. Che, A.J. Tench, Adv. Catal. 31 (1982) 77 and references therein. [37] M. Che, A.J. Tench, Adv. Catal. 32 (1983) 1 and references therein. [38] C.G. Vayenas, S. Bebelis, S. Ladas, Nature 343 (1990) 625. [39] C.G. Vayenas, S. Bebelis, I.V. Yentekakis, H.H. Lintz, Catal. Today 11 (1992) 303. [40] R.D. Shannon, Acta Cryst. A32 (1976) 751.