Surface electronic structure–catalytic activity relationship of partially reduced WO3 bulk or deposited on TiO2

Journal of Electron Spectroscopy and Related Phenomena 151 (2006) 128–134

Surface electronic structure–catalytic activity relationship of partially reduced WO3 bulk or deposited on TiO2 H. Al-Kandari a , F. Al-Kharafi a , N. Al-Awadi a , O.M. El-Dusouqui a , A. Katrib b,∗ a

b

Kuwait University, Department of Chemistry, P.O. Box 5969, Safat 13060, Kuwait LMSPC-UMR 7515 du CNRS-ECPM, University Louis Pasteur, 25, rue Becquerel, 67087 Strasbourg, France Received 20 June 2005; received in revised form 15 November 2005; accepted 15 November 2005 Available online 27 December 2005

Abstract Catalytic reactions of n-pentane and 1-pentene were carried out as a function of the reduction process of WO3 as bulk or supported on TiO2 . In situ characterization by XPS–UPS techniques of the different chemical species formed following the reduction processes was performed. At reduction temperatures up to 773 K, two distinct W5+ and W4+ states were obtained. The acid functions associated with W5+ , mainly of Br¨onsted type, enabled to isomerize only 1-pentene to unsaturated products. Dual metal–acid functions (bifunctional) of the W4+ state perform the isomerization processes of 1-pentene and n-pentane to isopentane in similar way to highly dispersed platinum on alumina catalysts. A conversion of 56% of n-pentane and a selectivity of 87% to isopentane were obtained at 623 K reaction temperature. © 2005 Elsevier B.V. All rights reserved. Keywords: XPS; UPS; WO3 ; Isomerization of n-pentane; 1-Pentene

1. Introduction Hydroisomerization of light fractions from normal paraffins is an important catalytic process used to improve the octane number of gasoline. For the same objective, alkenes such as pentenes are isomerized and incorporated in the gasoline pool. Pentenes are also used for the synthesis of (tert amyl methyl ether) TAME as well as for alkylation reactions [1–6]. In general, the most frequently used catalytic systems for the isomerization of C5 –C6 alkanes consist of Pt deposited on acidic alumina or zeolites. The catalytic process by which these systems function is interpreted in terms of bifunctional mechanism [7]. Dehydrogenation of alkane and the hydrogenation of the isomerized olefin take place on the finely dispersed Pt particles of metallic character. On the other hand, the acidic function assured by chlorinated alumina or zeolite, enables olefin isomerization via carbenium ion mechanism. In the case of olefins such as 1-pentene, different isomerization reactions are possible: (i) double bond isomerization (DbI), (ii) skeletal isomerization (SkI), (iii) direct cracking of pentene (Cr) and (iv) dimerization (and

∗

Corresponding author. Tel.: +33 3 902 427 56; fax: +33 3 902 727 61. E-mail address: [email protected] (A. Katrib).

0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.11.007

oligomerization) (Dim) may take place, depending on the catalyst acid function(s) strength(s) [5–6]. In previous works, some characterization and catalytic properties of a bifunctional MO2 (Hx )ac phase system (M = Mo, W) have been presented [8–10]. In order to get closer correlation between catalytic activity and surface electronic structure, appropriate surface techniques and well-defined catalytic reactions have to be chosen. XPS–UPS are the most practical and convenient surface techniques for the determination of the chemical composition of the different species in the 10–15 monolayers from the surface. In the case of partially reduced WO3 by hydrogen, XPS enables to define the oxidation states of different tungsten sub-oxides. The presence of a metallic character as it is the case in WO2 , is observed in the UPS spectrum as a density of states (DOS) at the Fermi-level with a maximum at 0.7 eV [11]. Moreover, the presence of Br¨onsted acidic (–OH) groups on the surface is observed in the form of O1s at 531.6 eV as compared to the oxide O1s at ∼530.5 eV [8]. One of the main objectives of this work is to demonstrate the relationship between surface electronic structure and catalytic activity of a given chemical species. In the reduction process of WO3 by hydrogen, different tungsten sub-oxides, mainly W5+ and W4+ are formed as a function of reduction temperature

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increase. It is interesting to note that high resolution studies of the W(4f) energy region using 200 eV low energy synchrotron radiation of deposited 150 nm thin film of WO3 on Si(1 0 0) reveal the presence of sub-stoichiometric WO3−x (x < 1) structure as characterized by W(4f) binding energies [12]. This low intensity structure is assigned to oxygen vacancies on the sample surface, which leads to specific gas sensitivity of this compound. Taking into consideration the different electronic properties of tungsten sub-oxides, different catalytic behavior, if any, is expected to be observed. 1-Pentene and n-pentane molecules could be considered as good example in this respect. As mentioned above, isomerization reactions of 1-pentene to different olefins require acid function(s), while both metal and acid functions are required for the isomerization of n-pentane in terms of the bifunctional mechanism. Therefore, catalytic activity tests are performed by the introduction of each of the reactant at a time as a function of the reduction process of WO3 by hydrogen at different temperatures in similar way to the experimental procedure carried out in the characterization process. The supported WO3 on TiO2 has two main advantages: it increases the surface area of the bulk WO3 phase from 1–2 to 50 m2 /g, and adds mechanical strength required for practical use in industry. Moreover, deposition of W on TiO2 takes place in a homogeneous way as demonstrated by Hercules and co-workers using ISS technique [13]. Therefore, no direct interference of TiO2 , if any, is expected to take place with the reactant molecules.

284.8 eV. Binding energies are reported within an experimental error of ±0.2 eV. High resolution transmission electron microscopy measurements were conducted using a 3010 JEOL instrument.

2. Experimental

These values are in complete agreement with those reported in the literature [15–19]. In the following, we present the XPS–UPS spectra before and after hydrogen reductions at 673 and 773 K for 2 h each. Two spectral lines at 35.8 and 37.7 eV characteristics of WO3 state were observed prior to any reduction treatment for both bulk and the supported system (Fig. 1a). The calcination procedure of the deposited ammonium metatungstate on TiO2 at 773 K enabled to convert all the tungsten atoms in this salt to WO3 . The insulating properties of WO3 could be verified from the absence of any DOS structure at the Fermi level in the UPS spectrum (Fig. 2a). The O1s energy region consists of the main oxide oxygen spectral line at 530.5 eV (Fig. 3a). The presence of relatively low intensity shoulder at higher BE is attributed to the adsorption of various air oxygen compounds on the sample surface. Exposure of the sample to hydrogen at 673 K for 2 h results in its partial reduction to W2 O5 state as could be observed in the W(4f) energy region (Fig. 1b). A density of state at 0.7 eV, characteristic of the metallic properties of the WO2 phase is observed in the UPS spectrum following this reduction process (Fig. 2b). The presence of this DOS structure in relatively low intensity is attributed to the formation of limited number of WO2 structure on the sample surface. This low concentration of WO2 is not detected by XPS. Moreover, the O1s energy region shows the presence of a shoulder at ∼531.6 eV beside the main oxide oxygen O1s at 530.5 eV (Fig. 3b). This pronounced shoulder is attributed to the formation of Br¨onsted W–OH acid group(s) on the sample surface. The ratio of the O1s spectral line relative intensity attributed to the –OH group(s) to oxide oxygen is determined at 0.26. Further increase in the reduction temperature to

Bulk WO3 and the equivalent of five monolayers of tungsten trioxide were deposited on TiO2 using ammonium metatungstate (NH4 )6 H2 W12 O40 ·xH2 O (99.9%) were supplied by STREM Chemicals. TiO2 is Degussa P-25 (25% rutile) with pore volume of 0.5 cm3 /g and BET surface area of 50 ± 5 m2 /g. Supported catalyst is prepared by impregnating the appropriate amount of tungsten in ammonium metatungstate hydrate salt, following the method described by Pines et al. [14]. The catalytic experiments were performed using 100 mg of the catalyst in a fixed bed quartz reactor. The catalytic experiments were conducted in a pulse and time-on-stream conditions under continuous hydrogen and hydrocarbon reactant flow at the rate of 40 cm3 /min. The catalytic products were analyzed by gas chromatography using a 50 m (CP-SIL-5CP) column and a flame ionization detector. Characterization of the samples by XPS were conducted using VG Scientific ESCALAB-200 spectrometer. A Mg K␣ radiation source operating at a power of 300 W (15 kV, 20 mA) was applied. XPS data and curve fitting were analyzed using the ECLIPSE VG V 2.1 supplied with the instrument. Peaks are fitted in the symmetric mode. Gauss/Lorentz ratio is used taking into consideration the full width at half maximum, height ratio and peak center with the largest peak as a reference. UPS ˚ radiation of 21.217 eV were employed in He(I) resonance 584 A order to measure the VB energy region. Vacuum in the analysis chamber was below 7 × 10−9 mbar during all measurements. In situ reduction was carried out in high-pressure gas cell in the preparation chamber with hydrogen flow at 200 ml/min. Binding energies were based on the carbon contamination C1s at

3. Results and discussion 3.1. Characterization by XPS–UPS and HRTEM The XPS and UPS spectra of bulk WO3 and the equivalent of five monolayers of WO3 deposited on TiO2 were recorded before and following the in situ sample reduction by hydrogen at different temperatures. The sample is pressed in the form of a pellet and introduced inside the XPS–UPS spectrometer. The XPS of W4f, O1s, C1s and Ti2p as well as the UPS of the valence band (VB) energy regions were recorded. The different tungsten oxide states are characterized by the W(4f7/2, 5/2 ) spin–orbit components binding energies in eV obtained from curve fitting with FWHM in parenthesis as follows: Before treatment

673 K

4f5/2

4f5/2

4f7/2

773 K 4f7/2

4f5/2

4f7/2

WO3 37.7 (1.5) 35.6 (1.4) 37.7 (1.6) 35.6 (1.6) 37.7 (1.6) 35.6 (1.6) W2 O5 – – 36.7 (1.6) 34.5 (1.6) 36.7 (1.6) 34.4 (1.7) WO2 – – – – 34.9 (1.6) 32.8 (1.0)

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Fig. 1. The XPS spectra of the W(4f) energy region of the in situ reduction by hydrogen of commercial bulk WO3 at different temperatures each for 2 h: (a) as received; (b) 673 K; (c) 773 K.

773 K results in relative increase in the WO2 phase concentration with both W2 O5 and WO3 phases still present in the few monolayers measured in the XPS spectrum (Fig. 1c). A considerable increase in the relative intensity of the DOS structure at 0.7 eV, assigned to the ␲ band could be observed (Fig. 2c). On the other hand, a broad structure at higher BE involving the ␴ band expected at 1.8 eV could also be observed. The origin of this structure broadening is under exploration. Also, the shoulder

Fig. 2. The UPS spectra of the ␴ and ␲ bands of the in situ reduction by hydrogen of commercial bulk WO3 at different temperatures each for 2 h: (a) as received; (b) 673 K; (c) 773 K.

Fig. 3. The XPS of the O(1s) region of the in situ reduction by hydrogen of commercial bulk WO3 at different temperatures each for 2 h: (a) as received; (b) 673 K; (c) 773 K.

at 531.6 eV attributed to the –OH acid group(s) is still observed in the O1s spectrum with the same relative intensity (Fig. 3c). It is important to note that titanium dioxide used as a support for WO3 seems to be unaffected by the reduction process of supported sample at temperatures up to 773 K. The stability of this TiO2 phase could be verified from the Ti2p energy region which shows no change in the energy or Broadening (FWHM) of the two Ti(2p3/2 , 2p1/2 ) spectral lines at 459 and 464.7 eV before and after reduction treatments at 673 and 773 K (Fig. 4). The BE of these Ti(2p3/2 , 2p1/2 ) spin–orbit components are in agreement within experimental error with those reported in the literature [20]. The presence of both the metallic character in WO2 and Br¨onsted W OH acid functions on the same site on the sample surface could be summarized as follows: the reduction of WO3 to WO2 results in the presence of two free electrons per W atom in WO2 . These free electrons form ␲ and ␴ bonds between adjacent W W atoms placed along the C-axis of the deformed rutile structure of WO2 . Conjugation (resonance) interaction between these ␲ electrons produces a de-localized electronic band observed as a density of state and measured at 0.7 eV [11]. On the other hand, the presence of alternate ␴ bonds between W atoms placed along the C-axis and measured at 1.8 eV produces ˚ [21]. two different W W bond lengths at 2.475 and 3.096 A In fact, these alternate W W bond lengths could be observed in the HRTEM image of the partially reduced WO3 /TiO2 by hydrogen at 773 K for 12 h Fig. 5. The metallic character of WO2 enables to dissociate hydrogen molecules, used as reducing agent in this reduction process. Bonding of the resultant H atoms with surface oxygen results in the formation of Br¨onsted W OH group(s) on the sample surface. The presence of these –OH groups is observed in the form of O1s at 531.6 eV. The

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the reduction by hydrogen of WO3 at temperatures between 673 and 773 K clearly indicates that the sample surface is completely converted to the WO2 (Hx )ac phase at 673 K. Further increase in the reduction temperature results in relative increase in the bulk WO2 concentration. 3.2. Catalytic results and discussion

Fig. 4. The XPS spectra of the Ti(2p) energy region of the in situ reduction by hydrogen of WO3 /TiO2 at different temperatures each for 2 h: (a) as received; (b) 673 K; (c) 773 K.

existence of both metallic and acid functions on the same site is represented as WO2 (Hx )ac , in which x (1 ≤ x ≤ 2) designates the number of Br¨onsted acid (ac) W OH group(s). The stability of the O1s intensity ratio of the –OH to the oxide oxygen following

Fig. 5. High-resolution transmission electron micrograph of partially reduced WO3 /TiO2 at 773 K for 12 h.

3.2.1. Isomerization of 1-pentene As stated in Section 1, isomerization reactions of 1-pentene to different olefins require only acid function of different strengths depending on the type of the isomerization catalytic process. No catalytic activity was observed following the introduction of 1-pentene on WO3 /TiO2 sample at 291 K (Table 1). A conversion of 82% of 1-pentene reactant was obtained following catalyst sample exposure to hydrogen at 303 K for 2 h. The main catalytic products are double bond (Db.) trans- and cis-2pentenes isomers. This catalytic process requires relatively mild acid function strength. Further increase in the reduction temperature to 373 K for 2 h enables to increase the conversion to 94.4% producing the same Db. isomerates. This catalytic activity is attributed to the partial reduction of WO3 to lower valency WOx (2 < x < 3), most probably W5+ as observed by XPS. The presence of Lewis and more likely Br¨onsted acid function(s) on this partially reduced surface enable to perform this isomerization process of 1-pentene. The increase in the conversion as a function of time and reduction temperature is attributed to the relative increase in the concentration of this lower valency state of W5+ on the sample surface. The first appearance of n-pentane and isopentane products takes place following the exposure of the sample to hydrogen at 423 K for 2 h. Hydrogenation of 1-pentene (3%) to npentane is usually performed by a metallic function. Moreover, the formation of isopentane (6%) requires both metallic and acid functions in terms of the bifunctional mechanism. Consequently, the initial formation, in relatively low concentration, of the WO2 (Hx )ac state takes place on the sample surface at this reduction temperature. Further increase in the reduction (reaction) temperature results in considerable increase in the formation of isopentane and a substantial decrease in Db. isomerization products. To mention at this point that at reduction temperatures up to 623 K, the reduction and reaction temperatures are the same. However, at higher reduction temperatures, the reaction temperature is maintained at 623 K. Table 1 shows that a conversion of 100% to mainly isopentane (51.5%) and npentane (31.9%) were obtained at 773 K reduction temperature. This corresponds to the total conversion of the sample surface to WO2 (Hx )ac as verified by extended reduction time for more than 12 h with no change in the catalytic behavior. These results are in conformity with XPS–UPS and HRTEM measurements. These catalytic results are interpreted as follows: initially, exposure of the WO3 /TiO2 sample to hydrogen at 291 K did not show any catalytic activity towards 1-pentene reactant. The presence of Lewis and Br¨onsted acid functions, if any, following the sample reduction up to 291 K seems to be insufficient in order to initiate a catalytic activity for 1-pentene. This catalytic inactivity could be attributed to kinetic and thermodynamic restrictions.

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Table 1 Isomerization reactions of 1-pentene and products distribution following the reduction of WO3 /TiO2 at different temperatures each for 2 h Reaction temperature (K)

Conversion Methane Ethene Ethane Propene Propane Isobutene 1-Butene Butane trans-2-Butene cis-2-Butene 3Methyl-1butene Isopentane 2Me-1-butene Pentane trans-2-Pentene cis-2-Pentene 2Me-2-butene Hexane a

291

303

323

373

423

473

523

573

623

653

653a

723

0

82.16 0 0 0 0 0 0 0 0 0 0 0 0 0 0 73.95 26.05 0 0

90.66 0 0 0 0 0 0 0 0 0 0 0 0 0 0 77.04 22.96 0 0

94.43 0 0 0 0 0 0 0 0 0 0 0 0 0 0 67.65 24.95 7.40 0

94.39 0 0 0 0 0 0 0 0 0 0 0 6.44 2.07 3.06 55.27 23.77 9.39 0

99.33 0 0 0 0.13 0 13.81 3.54 0.63 0 0 0 46.07 2.63 1.90 4.52 2.30 10.42 14.05

100 0 0 0 0.63 0 24.74 4.23 0.31 1.37 0.84 0 44.14 1.76 1.43 1.19 0.63 6.07 12.67

100 0 0 0 2.37 0 22.83 8.14 0.79 2.98 1.91 0 35.61 3.19 1.48 1.97 1.10 9.54 8.1

99.26 0 0.24 0 3.91 0 16.54 9.70 1.30 3.80 2.61 0.63 31.92 4.65 2.07 3.14 1.83 12.18 5.48

99.33 0 0.24 0 3.70 0 18.35 8.43 1.10 3.31 2.36 0.59 34.72 4.01 2.10 2.66 1.60 10.55 6.27

99.29 0 0.3 0 3.78 0 14.75 7.78 1.11 2.96 2.03 0.59 37.67 4.49 2.26 2.93 1.70 11.82 5.82

100 0 0 0 2.81 0 4.44 4.36 0 0 0 0 45.99 8.38 3.55 5.40 3.13 21.94 0

773 100 0 0 0 0 2.60 2.47 0.47 1.72 0 0 0 51.48 1.53 31.90 0.95 0 4.15 2.72

After 12 h reduction.

However, a conversion of 26.5% were obtained at 291 K reaction temperature following the reduction of the sample surface to the WO2 (Hx )ac phase, as discussed later in this text (Table 2). Mild acid function(s), mainly Br¨onsted and probably Lewis types associated with tungsten sub-oxide(s) such as W5+ , obtained following the sample reduction at temperatures between 323 and 423 K, perform double bond isomerization of 1-pentane. The beginning of the reduction process towards WO2 seems to start at 423 K as can be deduced from the formation of isopentane and n-pentane in relatively low concentration. Complete surface conversion to the bifunctional WO2 (Hx )ac phase is obtained at 773 K

reduction temperature. Evolution of catalytic products distribution of 1-pentene as a function of tungsten phase changes on the WO3 /TiO2 sample surface is given in Fig. 6. Following total sample surface covering by the bifunctional WO2 (Hx )ac phase, it is of interest to study the catalytic behavior of this phase towards 1-pentene as a function of reaction temperature. Total conversion of 1-pentene molecules were obtained at reaction temperatures between 773 and 323 K (Table 2). However, products distribution depends on the reaction temperature due to thermodynamic and kinetic factors. Extensive hydrocracking products, mainly C1, C2 and C3 compounds were

Table 2 Products distribution of 1-pentene as a function of reaction temperature on partially reduced WO3 /TiO2 sample for 12 h at 773 K Reaction temperature (K)

Conversion Methane Ethene Ethane Propene Propane Isobutane 1-Butene Butane trans-2-Butene cis-2-Butene 3Me-1-butene Isopentane 2Me-1-butene Pentane trans-2-Pentene cis-2-Pentene 2Me-2-butene Hexane a

291

303

323

373

423

473

523

573

623

653

723

773a

26.48 0 0 0 0 0 0 0 0 0 0 0 0 0 0 68.56 31.44 0 0

95.89 0 0 0 0 0 0 0 0 0 0 0 0 0 0 74.86 25.14 0 0

100 0 0 0 0 0 0 0 0 0 0 0 0 0 0 77.0 23.0 0 0

100 0 0 0 0 0 0 0 0 0 0 0 0 0 44.34 40.69 14.97 0 0

100 0 0 0 0 0 0 0 0 0 0 0 4.70 0 88.81 4.52 1.98 0 0

100 0 0 0 0 0 3.10 0.66 0.750 0 0 0 25.91 0 64.87 1.47 0 2.09 1.90

100 0 0 0 0 0 2.87 0 0 0 0 0 42.18 0 54.95 0 0 0 0

100 0 0 0 0 0 0 0 0 0 0 0 48.21 0 51.79 0 0 0 0

100 00 0 0 0 0 0 0 0 0 0 0 57.82 0 40.42 0 0 1.77 0

100 0 0 0.22 0 1.05 0.70 0 1.02 0 0 0 56.26 0.68 38.38 0 0 1.70 0

99.55 2.61 0 3.73 0 10.0 3.96 0.60 6.21 0.36 0 0 39.79 1.55 25.71 1.34 0.86 3.28 0

100 19.63 0 14.57 0 18.49 4.18 1.24 5.60 0.58 0 0 17.72 1.52 11.65 1.12 0.76 2.93 0

After 12 h reduction.

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Table 3 Isomerization reaction of n-pentane and products distribution following the reduction of WO3 /TiO2 at different temperatures each for 2 h Reduction temperature (K)

Reaction temperature (K)

673

723

773

773a

0.6 100

16.2 59.7

32.3 16.4

62.3 3.1

50.4 29.1

35.0 65.3

28.5 87.5

16.1 96.6

2.8 100

0.7 100

Distribution (isomerization products, %) iC5 0 100

59.7

16.4

3.1

29.1

65.3

87.5

96.6

100

100

Distribution (cracking products, %) Methane 0 0 Ethane 0 0 Propane 0 0 Butane 0 0 Butene 0 0 Pentene 0 0 Hexane 0 0

1.4 5.4 8.9 7.6 0.8 14.5 1.7

16.9 26.6 18.9 10.4 2.0 8.0 0.9

36.6 40.0 15.0 4.3 0 1.1 0

8.1 19.4 21.6 16.2 0.3 4.4 0.8

1.7 5.7 9.5 11.4 0 5.6 0.8

0.5 0.8 3.3 5.6 0 1.5 0.8

0.1 0.2 0.9 1.5 0 0.6 0

0 0 0 0 0 0 0

0 0 0 0 0 0 0

623 Conversion Selectivity

a

0 0

723

693

652

623

573

523

After 12 h reduction.

obtained at 773 K reaction temperature. The maximum isomerization process of 1-pentene to isopentane takes place at 623 K reaction temperature. It is interesting to note that at 423 K reaction temperature, 88.8% of 1-pentene are hydrogenated to n-pentane as compared to only 3% obtained at the beginning of the reduction process at 423 K (Table 1). It is clear that the conversion of 1-pentene to isopentane follows the hydrogenation/dehydrogenation pathway. As a result, dehydrogenation of n-pentane seems to be the rate-determining step in the isomerization process to produce isopentane. Reaction mechanism study of these catalytic processes is the subject of separate study. To note also, that the Br¨onsted acid function of the WO2 (Hx )ac phase catalyze the Db. isomerization to trans- and cis-2-pentene at 291 K. Such catalytic process was not possible at the start of the reduction process of WO3 /TiO2 sample. This is due, most probably, to insufficient acid function(s), if any, in WO3 prior to any reduction treatment, which enable to perform the Db. isomerization of 1-pentene. It is worth mentioning that TiO2 alone did not show any catalytic activity for 1-pentene or n-pentane reactants.

3.2.2. Isomerization of n-pentane The first catalytic activity with a conversion of 0.6% of npentane to isopentane during the reduction process of WO3 /TiO2 was observed at 673 K (Table 3). This is to be compared with 423 K for 1-pentene. Difference in adsorption activation energies between the two reactants accounts for such behavior. Adsorption of 1-pentene on the surface of the bifunctional WO2 (Hx )ac phase is much easier due to the presence of a double bond. Following complete conversion of the sample surface to the WO2 (Hx )ac phase at 773 K, a conversion of 62.3% of n-pentane was obtained at 773 K reaction temperature. As was observed in the case of 1-pentene, hydrocracking species are dominant in this case due to thermodynamic effects. It is interesting to note that selectivity of n-pentane to isopentane on this WO2 (Hx )ac surface increases as the reaction temperature decreases. Moreover, a catalytic activity is observed at 523 K on this well-defined surface as compared to 673 K during the initial reduction process. This is attributed to the substantial increase in the catalytic active WO2 (Hx )ac sites present on the sample surface following the reduction of the sample at 773 K for 12 h. 4. Conclusion

Fig. 6. Conversion and products distribution of 1-pentene on WO3 /TiO2 as a function of reduction temperature.

Hydrogen reduction of bulk WO3 or the equivalent five monolayers of WO3 deposited on TiO2 as a function of time and temperature follows the WO3 → W2 O5 → WO2 {WO2 (Hx )ac } pathway as characterized by XPS–UPS and HRTEM. A complete conversion of the sample surface to WO2 phase is obtained following the sample reduction at 773 K for 12 h. The tungsten dioxide species has metallic character due to de-localized ␲ electrons on the W W atoms placed along the C-axis of the deformed rutile structure of this WO2 phase. This metallic character is observed as a density of states at the Fermi level and measured at 0.7 eV in the UPS–XPS spectra. Following the formation of WO2 , hydrogen is dissociated by the metallic function of this phase to produce Br¨onsted acid W OH group(s) on the sample surface. These –OH group(s) are identified in the form of O1s at 531.6 eV. As a result, a bifunctional (metal–acid)

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WO2 (Hx )ac phase is formed on the sample surface. The catalytic behavior of these different tungsten sub-oxides has been tested by using 1-pentene and n-pentane reactants. Introduction of these reactants in pulses on the WO3 /TiO2 sample following its exposure to hydrogen in similar manner followed for XPS–UPS characterization, enabled us to get a clear correlation between the surface electronic structure and its catalytic activity. It is revealed, that isomerization of 1-pentene to isopentane takes place via its hydrogenation to n-pentane as a first step in terms of a bifunctional mechanism. In this isomerization process, the dehydrogenation process is the rate-determining step. Acknowledgements The support by Kuwait University through research Grant # SC04/00. ANALAB and SAF Grant # GS01/01 is gratefully acknowledged. References [1] T. Li, S.T. Wong, M.C. Chao, H.P. Lin, C.Y. Mou, S. Cheng, Appl. Catal. A. 261 (2004) 211. [2] J. Salmones, R. Licona, J. Navarrete, P. Salas, J. Morales, Catal. Lett. 36 (1996) 135.

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