The multi-surface structure and catalytic properties of partially reduced WO3, WO2 and WC + O2 or W + O2 as characterized by XPS

Journal of Electron Spectroscopy and Related Phenomena 76 (1995) 195-200

The multi-surface structure and catalytic properties o f partially reduced W O 3, W O 2 and W C + 0 2 or W + 0 2 as characterized by XPS A. Katrib, F. Hemming, P. Wehrer, L. Hilaire and G. Maire LERCSI, URA 1498 CNRS-EHICS-ULP, 4, rue Blaise Pascal, 67070 Strasbourg, France

The XPS at different experimental conditions of WO3, WO2, W(0), WC, W(0) + 0 2 and WC + 0 2 are reported. Extensive hydrogenolysis reactions of 2-methylpentane )ielding methane a major product were observed on relatively pure W(0) and WC surfaces. Isomerization reactions for the same reactant were observed on WO2 or partially oxydized W(0) and WC surfaces, while bulk WO3 does not show any catalytic reactivity at the beginning of the reaction. The catalytic activity of WO2 is attributed to the presence of a certain density of states of the 5d, 6s electrons present at the Fermi-level on the W4+ cation and observed in the XPS of the valence band energy region of WO2. Such reduction by H2 and the hydrocarbon reactant of bulk WO3 or that present on W(0) or WC surfaces results in the presence of WO2 and to a lesser extent WO and W(0), which explains the isomerization reactions carded on these modified surfaces.

1. INTRODUCTION

Tungsten metal carbides such as WC and W2C were expected to replace supported noble metals known for their bifunctional catalytic activity in hydrogenolysis and isomerization reactions of alkanes. However, it has been observed that alkane hydrogenolysis reactions are predominant on relatively pure WC surfaces. On the other hand, isomerization reactions for the same alkane reactants were observed upon the introduction of oxygen on WC at temperatures equal or higher than 620 K [1-3]. Similar isomerization catalytic properties were observed at 603 K upon the partial reduction of bulk WO3 by H2 or partial oxidation of W(0) [4]. The modified catalytic properties upon the addition of oxygen on WC has been attributed to the formation of new species such as tungsten "oxyearbide" [5], or to the chemisorbed oxygen [2]. In order to obtain further informations about the nature of the active species responsible for isomerization reactions in the above mentioned systems in function of the different treatments, we have carried out a systematic study using the XPS of the different catalytic systems by varying the 0368-2048/95 $09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(95) 02451-4

experimental conditions in correlation with their catalytic activities. It is well established that in-situ argon ion bombardment could be used beside its sputtering effect as a reducing agent in order to obtain lower valency oxidation or even the elemental state of some transition metal oxide(s) in function of the exposure time of the sample to these energetic ions [6.7].

2. EXPERIMENTAL Tungsten trioxide, WO3 and dioxide, WO2 of high purity were obtained from STREM Chemicals. Tungsten powder and WC were prepared from WO3 following the procedure reported previously [8l. The partial oxidation of WC as well as W(0) by exposing the sample to a flux of He gas containing oxygen in variable amounts at given temperature(s) has also been reported. Catalytic reactions were camed out in a hydrogen flow system at atmospheric pressure. Reactions were performed at 603 K at a flow rate of 54 cc.min-~, 5 ~tl of the reactant (2-methylpentane, partial pressure = 6.8 tort) was injected into the

196

reactor. The reaction products were analysed by gas chromatography. The XPS spectra were obtained using an ESCA III, VG instrument with Al K s radiation. All the spectral line binding energies were referred to the Cls at 284.6 eV. Standard energy differences between the different tungsten oxides and FWHM of the W4f spectral lines were considered in the curve filling process of the W4f energy region of the systems studied in this work. Argon ion bombardment (3 KV, 30 ~A) was employed as sputtering and reducing agent. Binding energies are reported within an experimental error of + 0.2 eV.

3. RESULTS AND DISCUSSION

The XPS of the W4f and the valence band energy regions of tungsten powder, W(0), tungsten carbide, WC and tungsten trioxide, WO3 are reported in figures 1 and 2. A relatively small amount of WO3 could be observed in the W4f energy region of W(0) and WC (fig. la and lb) as well as the VB energy region in terms of O2p derived states at ~ 8 eV. The W4f spin-orbit components of WC are measured at 31.8 and 33.9 eV as compared to W(0) at 31.2 and 33.4 eV. The valence band energy region of WC (fig. 2b) is assigned in terms of the W5d and 6s derived states in the region between the Ef and 3.5 eV, while the region between 3.5 and 8 eV is assigned to the C2p and O2p derived states. The C2s band energy is at ~ 12.5 eV [91. That of carbon Is in WC is assigned at 282.7 eV (fig. 3). Commercial WO3 shows the presence of relatively low intensity spectral lines in the W4f energy region at 34.5 and 36.7 eV (fig. lc) which are attributed to "W20 5" as compared to the main lines at 35.5 and 37.7 eV of WO3. The VB energy region of WO3 (fig. 2c) consists mainly of the O2p derived states. The experimental IP's of these states are comparable to those reported previously I8,91. _

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Figure 1. The XPS of the W4f energy region of (a) W(0), Co) WC, (c) WO3

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Figure 2 : The XPS of the valence band energy region of (a) W(0), Co) WC, (c) WO3

197

behaviour in terms of reaction mechanisms of WO 2 and MoO 2 will be published [11]. _:

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3.1. Tungsten dioxide, WO 2 Tungsten dioxide is one of the intermediate states obtained upon the reduction of WO 3 by H 2 and hydrocarbon reactant(s) in the catalytic reactor at temperatures equal or higher than 603 K. Its exact role in the catalytic reactions studied in this work has to be determined_ The XPS of WO 2 powder shows the presence of WO 3 at 35.5 and 37.7 eV as well as WO 2 at 32.9 and 35.1 eV in the W4f energy region (fig. 4a). The presence of WO 3 is due to the partial oxidation of the surface layers(s) of WO 2 as a result of its exposure to air. It is interesting to note the presence of a band at the Fermi-level (fig. 5a) which is attributed to the two electrons 5d, 6s derived states present in WO 2 in similar way to the 4d, 5s in MoO 2 [10]. In situ reduction by H 2 at 623 K for 5 hours results in a considerable reduction of WO 3 to WO 2 as can be observed from the W4f and VB energy regions (fig. 4b and 5b). The Ar + bombardment for 5 minutes following the hydrogen treatment results in a considerable reduction to the elemental W(0) state as can be observed in the appearance of two lines at 31.2 and 33.4 eV (fig. 4c), and an increase in the density of states (DOS) at the Fermi-level (fig. 5c). It is interesting to note that we did not observe the Mo(0) state from MoO 2 under the same reduction treatment [11]. Catalytic reactions using 2-methylpentane reactant with a flux of H 2 at 623 K on WO 2 shows a considerable activity after an induction time with a very high selectivity in terms of isomerization products. This catalytic activity is attributed mainly to thepresence of a certain DOS at the Fermi-level " m W 4 + . A detailed description of the catalytic

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Figure 4 " The XPS of the W4f energy region of WO 2 (a) as received, (b) in situ reductioon by H 2 for 5 hrs at 623 K, (c) At+ bomberdment of (b) for 5 min

3.2. WC + 0 2 The partial oxidation of WC in a controlled way at 200K followed by temperature increase to 373 K results in the formation of WO 3 beside WC as can be observed from the two spectral lines at 35.5 and 37.7 eV (fig. 6a). The presence of WO 3 could also be observed from the relative increase in the O2p derived states intensity in the VB energy region (fig. 7a). In-situ reduction by H 2 at 623 K in similar way to what carried out in the catalytic reactor results in a reduction of WO 3 to WO 2 and "WO" (figs. 6b and 7b). The catalytic reactions carried on this system using 2-methylpentane yields 3-methylpentane as a major isomerization product with a high activity and selectivity [4].

198

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Figure 5 " The XPS of VB energy region of WO 2 (a) as received, (b) in situ reduction by H 2 for 5 hrs at 623 K, (c) Ar + bombardment of(b) for 5 min

3.3. W + 0 2

The partial oxidation of W(0) powder prepared by H 2 reduction of WO 3 at 930 K followed by sample exposure to oxygen in a similar way to WC results also in the formation of WO 3 beside W(0) as can be observed in the W4f energy region (fig. ga). The absence of the C2p derived states at ~ 3.5 eV could be observed in the VB energy region (fig. 8a). Similarly, the reduction by H 2 at the same experimental conditions as WC + 0 2 leads to the formation of mainly W(0) (figs. 8b and 9b). Moreover, the catalytic activity of this modified W(0) surface by oxygen is comparable in terms of isomerization products to what observed previously in the ease of WC + 0 2 using the same 2methylpentane reactant [4].

3.4. W O 3 + H 2

The catalytic properties and XPS of bulk WO 3 following its treatment by hydrogen at different

Figure 6 • The XPS of the W4f energy region of WC + 0 2 (a) at 373 K, (b) in-situ reduction by H 2 at 623 K for 45 min

experimental conditions have been considered following the fact that isomerization reactions of 2methylpentane were observed on WC or W(0) surfaces only after their partial oxidation by oxygen where WO 3 constitutes the major species present on these surfaces. The reduction of WO 3 by H2, where WO 2 is formed at the early stages of reduction certainly plays an important role in this catalytic activity. In-situ reduction by H 2 of bulk WO 3 at 603 K for 12 hours shows the presence of WO 2 and W205 with very small amount of W(0) (fig. 10a). The presence of a band with a certain DOS at the Fermi-level (fig. 1 la) is attributed to these lower valency WO x (x _< 2) states as well as W(0). The presence of 2-methylpentane with H 2 at the same temperature seems to increase the reduction process of WO 3 to W(0) (figs. 10b and llb). Catalytic reactions of 2-methylpentane on WO 3 at 603 K shows no activity. However, the activity increases to reach a maximum within ~ 5 hours under H 2 at 700 K, while the selectivity remains

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Figure 7 " The XPS of the VB energy region of WC + 0 2 (a) at 373 K, (b) in-situ reduction by H 2 at 623 K for 45 min

Figure 9 " The XPS of the VB energy region of W + 0 2 (a) oxidation of W(0) at 1 atm pressure for 1 hour, (b) in-situ reduction by H 2 of (a) at 623 K for 45 min

high. On another experiment, it has been observed that the selectivity in isomerization decreases when the reduction temperature of w e 3 increased [41. Such a process is certainly associated with a different stoichiometric ratio between w e x and W(0) on the surface.

4. CONCLUSION

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Figure 8 • The XPS of the W4f energy region of W + 0 2 (a) oxidation of W(0) at 1 atm pressure for l hour, (b) in-situ reduction by H 2 of (a) at 623 K for 45 min

On that basis of the XPS spectra of different tungsten oxides and carbide obtained at different experimental conditions in correlation with the catalytic activities of these systems using 2methylpentane as a reactant it is concluded that extensive hydrogenolysis reactions with methane as the major product were observed on relatively pure W(0) or WC surfaces. The presence of a small amount of w e 3 in these systems does not affect this catalytic activity. On the other hand, bulk w e 3 is not active at the beginning of the reaction, while bulk w e 2 shows a relatively high activity and selectivity in isomerization products. The catalytic activity of w e 2 is attributed to the presence of a certain DOS of the two electrons 5d, 6s at the

200 Fermi-level of W4+ cation. Such an electron density is not present in the case of W6+ of the bulk WO3. This is why partially oxidized WC and W(0) surfaces activities increase with time under H2 in terms of isomerization reactions due to the reduction of WO3 to lower valency WOx (x < 2) oxidized states as well as W(0). The catalytic behaviour of these surfaces in terms of the nature and relative concentration of the isomerization products such as 3-methylpentane, n-hexane and 2,3-dimethylbutane depends on the stoichiometric ration between these WOx oxides and W(0). In others words, the nature and the number of the relatively free 5d, 6s electrons present on the tungsten cation plays the major role in the isomerization catalytic activities of the partially oxidized WC and W(0) surfaces. 27.00

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REFERENCES

Figure 10 : The XPS of the W4f energy region of WO3 (a) in-situ reduction by H 2 at 603 K for 12 hrs, (b) after reduction og WO3 by a mixture of H2 and 2-methylpentane at 603 K

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Figure 11 • The XPS of the VB energy region of WO3 (a) in-situ reduction by H2 at 603 K for 12 hrs, (b) after reduction og WO3 by a mixture of H2 and 2-methylpentane at 603 K

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