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Active sites of the isomerization of n-butane over oxygen-modified molybdenum carbide and molybdenum oxides
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
2369
Active Sites of the Isomerization of n-Butane over Oxygen-modified Molybdenum Carbide and Molybdenum Oxides S. Liebig a, T. G e r l a c h b, Kh. D o u k k a l i a, and W. Grtinert a aLehrstuhl fur Technische Chemie, Ruhr-Universit~it Bochum, P.O. Box 102148, D-44780 Bochum, Germany blnstitute of Applied Chemistry Berlin-Adlershof, Richard-Willst~itter-StraBe 12, D- 12484 Berlin, Germany The n-butane isomerization was studied at 623 K over MoO3 and related oxides. The catalysts were analyzed by XRD and XPS during transient changes of the isomerization selectivity. Catalytic and characterization data gave no indication that Mo oxycarbide or MoO2 provide isomerization sites. With triclinic Mo4Oll, high selectivity appeared without pronounced transient behaviour. The active site is suggested to be an oxide structure of high-valence Mo. 1. INTRODUCTION Mo and W carbides have been studied over years as substitutes for noble metals in catalysis [ 1]. It is known that attractive alkane isomerization catalysts are obtained by pretreating the carbides in oxygen [2-4]. The isomerization selectivity develops in transient formation periods during interaction of the O-treated surfaces with the HE/alkane feed. Similar transient selectivity patterns were shown to result from interaction of unsupported MoO3 with n-alkanes [5,6]. The alkane isomerization site is, however, still a matter of debate. Bifunctional catalysis was proposed on the basis of mechanistic studies [2], with (de)hydrogenation occurring over remaining carbide and alkene isomerization over W (Mo) oxide species. Altematively, interaction of Mo (W) oxide species with the hydrocarbon under reaction conditions was proposed to result in the formation of an oxycarbide providing the isomerization sites. Evidence from XRD, electron microscopy and XPS was presented to support this view [5]. Finally, the isomerization activity was attributed to MoO2 formed in the reduction of MoO3 [7]. This paper reports a study of the n-butane isomerization over MoO3 and related materials. The state of the catalyst during the transient formation period was monitored by XRD and XPS. Transitional Mo oxides were employed as starting materials to find relations between the oxidation state of the initial surface and the length of the formation period.
2. EXPERIMENTAL Materials. Mo oxides of different origin were employed: MoO3 and MoO2 from Aldrich, MoO3 from Riedel de Haen. Mo2C was made as recommended in [1] (temperature-programmed carbidization in flowing CH~H2, HE treatment at 873 K to remove surface carbon). After purging, the carbide was treated with 2 % o2/ne (800 K, 15 min; --~ MoEC/O). M09026, M08023, monoclinic and triclinic MO4Oll were obtained by heating mixtures of MoO3 and MoO2 in evacuated quartz ampoules. MoO3/MoO2 molar ratio y, reaction temperature (TR) and time (tR) were y -- 8, TR - 1023 K, tR - 4 d for MO9023, (7; 973 K; 3 d) for MosOll, (3, 823 K, 11 d) for monoclinic Mo4Oll, and (3; 923 K; 4 d) for triclinic Mo4Oll. The XRD pat-
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formation rate O
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Fig. 1. Development of activity and isobutane selectivity in the interaction of n-butane/H2 with Mo oxides at 623 K. Panel A: Transients, a, a', b, b' - MOO3, c - Mo2C/O, d - MOO3; panel B - comparison of maximum isobutane selectivities and formation rates terns showed the presence of the expected phases, but only triclinic Mo40~lwas pure. A MoO3 reflectance (d=231 pm) was present in the diffractograms of Mo9026 and Mo8023 at low intensity (4 and 8 % of major product reflectance), and in that of monoclinic Mo40~ at significant intensity (58 % of major product reflectance). A 0.5 wt.-% Pt/AI203 catalyst made by impregnation of y-AI203 (225 m2/g) with H2PtC16 was used as a reference sample.
Catalysis. The n-butane isomerization was studied in a flow reactor at 623 K. The reactant (10 % n-butane, 15 % Ar, balance H2) was fed over 0.5 (or 0.1) g of catalyst at 100-120 ml/ min, which gives a WHSV of --3 h -l (Mo2C/O --18 h-l). The n-butane conversion (X) was <10 % except if stated otherwise. It was mostly determined from the analysis of the products, but the justification for this procedure was checked with runs at X > 5 %. The isobutane selectivity is the ratio between rates of isobutane formation and n-butane consumption.
Characterization. XRD was recorded with a Philips PW 1050 instrument (CuKtx radiation). XPS was measured with a Leybold LHS 10 spectrometer (MgKtx radiation). Analysis of peak shapes was performed with the software package "Macfit" [8], with Shirley-type backgrounds and Voigt-type line shapes. Component line widths were determined from MoO3 and kept constant for all oxidation states except if the signal shape gave other indications. Used catalysts were passivated at 295 K with 02 leaked into flowing Ar and handled in air prior to XPS or XRD analysis. BET surface areas were estimated using a one-point BET technique. 3. RESULTS AND DISCUSSION Fig. 1 presents catalytic results obtained during the interaction of oxidic Mo surfaces with the n-butane/H2 feed. Panel A shows the transient formation process: both n-butane conversion X and isomerization selectivity increase gradually to reach maximum values. With MOO3, selectivities > 90 % are attained and held over a prolonged reaction period (curves a, a'). With the oxygen-treated Mo carbide, the transient is similar to that of M003 (curves c, c'- at increased
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Fig. 3(B). Development of isobutane selectivity during experiment shown in Fig 3(A)
WHSV, vide supra). The peak selectivity is, however, only 47 %, and hydrogenolysis is never suppressed. Curves d , d' report the behaviour of MOO2. The sample was reduced in H2 (15 % in Ar) prior to catalysis (773 K, 150 min) to remove Mo(VI) species covering its surface in the as-received state. The XP spectrum of MoO2 treated under these conditions is shown in Fig. 4 (Panel A). It is very close to a reference spectrum published in [9], which means that the procedure adopted removed Mo(VI) almost completely without overreducing the material. The complex form of the spectrum is discussed in [9, 10]. The catalytic results (Fig. 1) show this material to possess almost only hydrogenolysis activity (maximum isomerization selectivity - 8 %). During the run, the BET surface of MoO2 increased from 5.5 to 8.0 m2/g. Curve b in Fig. 1A reports an run with the same MoO3 as curve a, but with a micro-leak in the gas tubing (removed for run of curve a). Curve b is instructive since it shows all phases of the selectivity transient as seen in curve a on a different time scale. The selectivity decrease after the maximum is more pronounced. The MoO3 BET surface increased from 0.55 to 22 m2/g during this experiment. While this curve indicates the effect of experimental problems on reproducibility, the effect of the sample history and origin was much more pronounced. With Aldrich MOO3, we did not find significant n-butane conversion under our conditions. The difference arises, probably, from the mild reduction conditions, which will result in a dramatic influence of the surface-defect density on the reduction kinetics. Fig. 1B summarizes maximum isobutane selectivities and formation rates. In the MoO3 runs compared, the isobutane formation rate was equal, at slightly different selectivities. A similar isobutane formation rate was found with Mo2C/O, but much butane was lost by hydrogenolysis. Pt/AI203 was very active (X = 22 --+ 12 % in 22 h), but the selectivity did not exceed 3 %. Fig. 2 demonstrates that the transfer of reacted catalysts through the isomerization sites. It shows the development of conversion derived material after the transient (cf. Fig. 1 A, curve a) maximum by contact with air (1 h, 70 ml/min) at 295, 473,
air should not have destroyed and selectivity of the MoO3had been interrupted at the or 673 K. Up to 473 K, air
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Figure 3(A). XRD of MoO3 after different interaction times with n-butane/H2 at 623 K treatment has little effect on the selectivity. The conversion transient may be due to O adsorbed on the sites, which would then be blocked but not destroyed. Figure 3 (A) reports XRD recorded after the initial MoO3 had been contacted with the n-butane/H2 feed for different times and compares them with those of MoO3 and MOO2. The diffractograms are normalized to the most intense reflectance by the factors given in the figure. The traces denoted by "xy min" are from a run where the catalyst was re-filled into the reactor after acquisition of the diffractograms. Panel B (previous page) shows the development of selectivity in this run as compared with a subsequent reference run without interruptions. Although the selectivity achieved is somewhat lower in the interrupted run, the overall tendency agrees with the reference run despite the strong upset of the experimental regime the XRD analysis including handling in air. Immediately after re-start, the selectivity is attenuated in most cases, but not suppressed. After the first interruption, the reflectances of MoO3 are strongly enhanced (panel A), but no new signals emerge. With ongoing time, the crystallinity decreases, the MoO3 signals disappear, and MoO2 becomes detectable. In the vicinity of the selectivity maximum, new broad reflectances appear at 20=44.8 ~ 29.0 o and 14.6 o (d=203, 309, and 635 pm). In the "interrupted" run, these reflectances decreased relative to those of MoO2 after the selectivity maximum. However, in other runs also included in Fig 3 (A), where XRD was performed only at or after the selectivity maximum, the intensity at 44.8 o was rather enhanced relative to those of MoO2 after the selectivity maximum. XPS of the MoO3 derived catalyst at and after the selectivity maximum is presented in Fig. 4. The Mo 3d signals, which were analyzed according to the principles outlined in [ 10], indicate the presence of Mo(VI), Mo(V), isolated and paired Mo(IV) in both cases. No carbidic Mo, which is expected at 228.2-5 eV, is detected. Analogously, in the C ls region, no carbide carbon can be observed. This is highly relevant, since if the oxycarbide is an interstitial compound, carbide C should be present. The oxycarbide surface might have been oxidized during handling in air, but this would contradict its role as isomerization site (cf. Fig. 2). The
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Figure 4. XPS of MoO3 after different interaction times of MoO3 with n-butane/H2 at 623 K C ls region contains a signal at 287-8 eV, which is oxidized carbon. The corresponding O signal is found at =532 eV, where O in oxycarbide has been assumed to appear [5]. It should be noted that it is difficult to make specific assignments in this binding-energy region. Figure 5 presents catalytic results obtained with the transitional Mo oxides. Generally, transient behaviour, activities and selectivities resemble those found with MOO3. Remarkably, in two cases the transient behaviour was clearly affected by using partially reduced starting material: With monoclinic Mo4OII, a selectivity of 80 % was attained after 70 min, with MoO3 after 160 min. With triclinic Mo4Oll, however, which was the material of the best purity, there was no pronounced selectivity transient at all: High isomerization selectivity was found from the very beginning. During these experiments, the BET surface of Mo9026 increased from 1.6 to 8.1m2/g, that of triclinic Mo4Oll from 0.7 to 4.8 m2/g. The catalytic phenomena observed by us upon interaction of n-butane with MoO3 agree with earlier results with several hydrocarbons [2-7] including n-butane [ 11 ]. Our characterization data, however, are at variance with some views expressed in the literature regarding the active sites. This cannot be ascribed to the fact that in our study the reacted samples have been handled in air before analyzing them by XPS or XRD: Fig. 2 shows that the active site survives even an air treatment at 473 K. In our XRD work with used catalysts, reflectances not attributable to well-defined Mo oxides were observed as reported by Delporte et al. [5], who ascribed them to a Mo oxycarbide. However, while two of our reflectances gave d spacings close to those reported in [5], the third one was different (d=309 pm; [5]: 410 pm). We did not find a clear correlation between the intensity of these signals and the selectivity (cf. Fig. 3). While any relation between intensity of reflectances and quantity of a phase may be subject to many uncertainties (e.g. differ-
2374 10
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ences in crystallinity, crystal size and shape), our results even do not sugges~t a correlation between the intensity of Mo oxx~rbide reflectances and selectivity. In the XPS y of reacted samples, no indication of a phase that might contain interstitial carbide was found. Since the isomerization site is tolerant towards handling in air, this is at variance with the oxycarbide being the isomerization site With transitional Mo oxides as starting materials, it was possible to decrease the length of the selectivity transient or avoid it at all (Fig. 5). While the decrease may be also due to accelerated reduction (increased density of structural defects), the immediate appearance of isomerization selectivity with triclinic Mo4OII suggests that the isomerization site is an oxide structure. Immediate appearance of isomerization selectivity was also observed by Matsuda et al. after mild reduction of MoO3 with H2 [12]. We propose, therefore, that the active site contains Mo ions in high oxidation states, possibly in a mixed-valence ensemble.
4. CONCLUSIONS Interaction of MoO3 with n-butane/H2 at 623 K leads to gradual development of a highly selective isomerization catalyst. The isomerization site survives contact with air under ambient conditions. XRD gives indications of oxycarbide formation, but no correlation with the isomerization selectivity. XPS provides no indication for the presence of oxycarbide after contact with air. High isomerization selectivity- without transient increase- is provided by starting with triclinic MO4Oll instead of MoO3. On this basis, the isomerization site is suggested to be an oxide structure with Mo in high oxidation state. ACKNOWLEDGEMENTS We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft. REFERENCES 1. J.S. Lee, S.T. Oyama, M. Boudart, J. Catal. 106 (1987), 125, 133. 2. F.H. Ribeiro, R.A. Dalla Betta, M. Boudart, et al., J. Catal. 130 (1992) 86. 3. M..J. Ledoux, C. Pham-Huu, H. Dunlop, J. Guille, Stud. Surf. Sci. Catal. 75 (1993) 955. 4. A. Frennet, G. Leclercq, L. Leclercq, et al., Stud. Surf. Sci. Catal. 75 (1993) 927. 5. P. Delporte, F. Meunier, C. Pham-Huu, M.J. Ledoux, et al., Catal. Today 23 (1995) 251. 6. P. DelGallo, F. Meunier, C. Pham-Huu, et al., Ind. Eng. Chem. Res. 36 (1997) 4166. 7. A. Katrib, P. Leflaive, L. Hilaire, G. Maire, Catal. Lett. 38 (1996) 95. 8. H. G. Boyen, "Macfit", available at the University of Basle, Institute of Physics. 9. B. Brox, I. Olefjord, Surf. Interface Anal. 13 (1988), 3. 10. W. Grtinert, A. Yu. Stakheev, R. Feldhaus, et al., J. Phys. Chem. 95 (1991), 1323. 11. M.J. Ledoux, F. Meunier, B. Heinrich, et al., Appl. Catal. A181 (1999), 157. 12. T. Matsuda, H. Shiro, H. Sakkagami, N. Takahashi, Catal. Lett. 47 (1997), 99.
2369
Active Sites of the Isomerization of n-Butane over Oxygen-modified Molybdenum Carbide and Molybdenum Oxides S. Liebig a, T. G e r l a c h b, Kh. D o u k k a l i a, and W. Grtinert a aLehrstuhl fur Technische Chemie, Ruhr-Universit~it Bochum, P.O. Box 102148, D-44780 Bochum, Germany blnstitute of Applied Chemistry Berlin-Adlershof, Richard-Willst~itter-StraBe 12, D- 12484 Berlin, Germany The n-butane isomerization was studied at 623 K over MoO3 and related oxides. The catalysts were analyzed by XRD and XPS during transient changes of the isomerization selectivity. Catalytic and characterization data gave no indication that Mo oxycarbide or MoO2 provide isomerization sites. With triclinic Mo4Oll, high selectivity appeared without pronounced transient behaviour. The active site is suggested to be an oxide structure of high-valence Mo. 1. INTRODUCTION Mo and W carbides have been studied over years as substitutes for noble metals in catalysis [ 1]. It is known that attractive alkane isomerization catalysts are obtained by pretreating the carbides in oxygen [2-4]. The isomerization selectivity develops in transient formation periods during interaction of the O-treated surfaces with the HE/alkane feed. Similar transient selectivity patterns were shown to result from interaction of unsupported MoO3 with n-alkanes [5,6]. The alkane isomerization site is, however, still a matter of debate. Bifunctional catalysis was proposed on the basis of mechanistic studies [2], with (de)hydrogenation occurring over remaining carbide and alkene isomerization over W (Mo) oxide species. Altematively, interaction of Mo (W) oxide species with the hydrocarbon under reaction conditions was proposed to result in the formation of an oxycarbide providing the isomerization sites. Evidence from XRD, electron microscopy and XPS was presented to support this view [5]. Finally, the isomerization activity was attributed to MoO2 formed in the reduction of MoO3 [7]. This paper reports a study of the n-butane isomerization over MoO3 and related materials. The state of the catalyst during the transient formation period was monitored by XRD and XPS. Transitional Mo oxides were employed as starting materials to find relations between the oxidation state of the initial surface and the length of the formation period.
2. EXPERIMENTAL Materials. Mo oxides of different origin were employed: MoO3 and MoO2 from Aldrich, MoO3 from Riedel de Haen. Mo2C was made as recommended in [1] (temperature-programmed carbidization in flowing CH~H2, HE treatment at 873 K to remove surface carbon). After purging, the carbide was treated with 2 % o2/ne (800 K, 15 min; --~ MoEC/O). M09026, M08023, monoclinic and triclinic MO4Oll were obtained by heating mixtures of MoO3 and MoO2 in evacuated quartz ampoules. MoO3/MoO2 molar ratio y, reaction temperature (TR) and time (tR) were y -- 8, TR - 1023 K, tR - 4 d for MO9023, (7; 973 K; 3 d) for MosOll, (3, 823 K, 11 d) for monoclinic Mo4Oll, and (3; 923 K; 4 d) for triclinic Mo4Oll. The XRD pat-
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Fig. 1. Development of activity and isobutane selectivity in the interaction of n-butane/H2 with Mo oxides at 623 K. Panel A: Transients, a, a', b, b' - MOO3, c - Mo2C/O, d - MOO3; panel B - comparison of maximum isobutane selectivities and formation rates terns showed the presence of the expected phases, but only triclinic Mo40~lwas pure. A MoO3 reflectance (d=231 pm) was present in the diffractograms of Mo9026 and Mo8023 at low intensity (4 and 8 % of major product reflectance), and in that of monoclinic Mo40~ at significant intensity (58 % of major product reflectance). A 0.5 wt.-% Pt/AI203 catalyst made by impregnation of y-AI203 (225 m2/g) with H2PtC16 was used as a reference sample.
Catalysis. The n-butane isomerization was studied in a flow reactor at 623 K. The reactant (10 % n-butane, 15 % Ar, balance H2) was fed over 0.5 (or 0.1) g of catalyst at 100-120 ml/ min, which gives a WHSV of --3 h -l (Mo2C/O --18 h-l). The n-butane conversion (X) was <10 % except if stated otherwise. It was mostly determined from the analysis of the products, but the justification for this procedure was checked with runs at X > 5 %. The isobutane selectivity is the ratio between rates of isobutane formation and n-butane consumption.
Characterization. XRD was recorded with a Philips PW 1050 instrument (CuKtx radiation). XPS was measured with a Leybold LHS 10 spectrometer (MgKtx radiation). Analysis of peak shapes was performed with the software package "Macfit" [8], with Shirley-type backgrounds and Voigt-type line shapes. Component line widths were determined from MoO3 and kept constant for all oxidation states except if the signal shape gave other indications. Used catalysts were passivated at 295 K with 02 leaked into flowing Ar and handled in air prior to XPS or XRD analysis. BET surface areas were estimated using a one-point BET technique. 3. RESULTS AND DISCUSSION Fig. 1 presents catalytic results obtained during the interaction of oxidic Mo surfaces with the n-butane/H2 feed. Panel A shows the transient formation process: both n-butane conversion X and isomerization selectivity increase gradually to reach maximum values. With MOO3, selectivities > 90 % are attained and held over a prolonged reaction period (curves a, a'). With the oxygen-treated Mo carbide, the transient is similar to that of M003 (curves c, c'- at increased
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Fig. 3(B). Development of isobutane selectivity during experiment shown in Fig 3(A)
WHSV, vide supra). The peak selectivity is, however, only 47 %, and hydrogenolysis is never suppressed. Curves d , d' report the behaviour of MOO2. The sample was reduced in H2 (15 % in Ar) prior to catalysis (773 K, 150 min) to remove Mo(VI) species covering its surface in the as-received state. The XP spectrum of MoO2 treated under these conditions is shown in Fig. 4 (Panel A). It is very close to a reference spectrum published in [9], which means that the procedure adopted removed Mo(VI) almost completely without overreducing the material. The complex form of the spectrum is discussed in [9, 10]. The catalytic results (Fig. 1) show this material to possess almost only hydrogenolysis activity (maximum isomerization selectivity - 8 %). During the run, the BET surface of MoO2 increased from 5.5 to 8.0 m2/g. Curve b in Fig. 1A reports an run with the same MoO3 as curve a, but with a micro-leak in the gas tubing (removed for run of curve a). Curve b is instructive since it shows all phases of the selectivity transient as seen in curve a on a different time scale. The selectivity decrease after the maximum is more pronounced. The MoO3 BET surface increased from 0.55 to 22 m2/g during this experiment. While this curve indicates the effect of experimental problems on reproducibility, the effect of the sample history and origin was much more pronounced. With Aldrich MOO3, we did not find significant n-butane conversion under our conditions. The difference arises, probably, from the mild reduction conditions, which will result in a dramatic influence of the surface-defect density on the reduction kinetics. Fig. 1B summarizes maximum isobutane selectivities and formation rates. In the MoO3 runs compared, the isobutane formation rate was equal, at slightly different selectivities. A similar isobutane formation rate was found with Mo2C/O, but much butane was lost by hydrogenolysis. Pt/AI203 was very active (X = 22 --+ 12 % in 22 h), but the selectivity did not exceed 3 %. Fig. 2 demonstrates that the transfer of reacted catalysts through the isomerization sites. It shows the development of conversion derived material after the transient (cf. Fig. 1 A, curve a) maximum by contact with air (1 h, 70 ml/min) at 295, 473,
air should not have destroyed and selectivity of the MoO3had been interrupted at the or 673 K. Up to 473 K, air
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Figure 3(A). XRD of MoO3 after different interaction times with n-butane/H2 at 623 K treatment has little effect on the selectivity. The conversion transient may be due to O adsorbed on the sites, which would then be blocked but not destroyed. Figure 3 (A) reports XRD recorded after the initial MoO3 had been contacted with the n-butane/H2 feed for different times and compares them with those of MoO3 and MOO2. The diffractograms are normalized to the most intense reflectance by the factors given in the figure. The traces denoted by "xy min" are from a run where the catalyst was re-filled into the reactor after acquisition of the diffractograms. Panel B (previous page) shows the development of selectivity in this run as compared with a subsequent reference run without interruptions. Although the selectivity achieved is somewhat lower in the interrupted run, the overall tendency agrees with the reference run despite the strong upset of the experimental regime the XRD analysis including handling in air. Immediately after re-start, the selectivity is attenuated in most cases, but not suppressed. After the first interruption, the reflectances of MoO3 are strongly enhanced (panel A), but no new signals emerge. With ongoing time, the crystallinity decreases, the MoO3 signals disappear, and MoO2 becomes detectable. In the vicinity of the selectivity maximum, new broad reflectances appear at 20=44.8 ~ 29.0 o and 14.6 o (d=203, 309, and 635 pm). In the "interrupted" run, these reflectances decreased relative to those of MoO2 after the selectivity maximum. However, in other runs also included in Fig 3 (A), where XRD was performed only at or after the selectivity maximum, the intensity at 44.8 o was rather enhanced relative to those of MoO2 after the selectivity maximum. XPS of the MoO3 derived catalyst at and after the selectivity maximum is presented in Fig. 4. The Mo 3d signals, which were analyzed according to the principles outlined in [ 10], indicate the presence of Mo(VI), Mo(V), isolated and paired Mo(IV) in both cases. No carbidic Mo, which is expected at 228.2-5 eV, is detected. Analogously, in the C ls region, no carbide carbon can be observed. This is highly relevant, since if the oxycarbide is an interstitial compound, carbide C should be present. The oxycarbide surface might have been oxidized during handling in air, but this would contradict its role as isomerization site (cf. Fig. 2). The
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Figure 4. XPS of MoO3 after different interaction times of MoO3 with n-butane/H2 at 623 K C ls region contains a signal at 287-8 eV, which is oxidized carbon. The corresponding O signal is found at =532 eV, where O in oxycarbide has been assumed to appear [5]. It should be noted that it is difficult to make specific assignments in this binding-energy region. Figure 5 presents catalytic results obtained with the transitional Mo oxides. Generally, transient behaviour, activities and selectivities resemble those found with MOO3. Remarkably, in two cases the transient behaviour was clearly affected by using partially reduced starting material: With monoclinic Mo4OII, a selectivity of 80 % was attained after 70 min, with MoO3 after 160 min. With triclinic Mo4Oll, however, which was the material of the best purity, there was no pronounced selectivity transient at all: High isomerization selectivity was found from the very beginning. During these experiments, the BET surface of Mo9026 increased from 1.6 to 8.1m2/g, that of triclinic Mo4Oll from 0.7 to 4.8 m2/g. The catalytic phenomena observed by us upon interaction of n-butane with MoO3 agree with earlier results with several hydrocarbons [2-7] including n-butane [ 11 ]. Our characterization data, however, are at variance with some views expressed in the literature regarding the active sites. This cannot be ascribed to the fact that in our study the reacted samples have been handled in air before analyzing them by XPS or XRD: Fig. 2 shows that the active site survives even an air treatment at 473 K. In our XRD work with used catalysts, reflectances not attributable to well-defined Mo oxides were observed as reported by Delporte et al. [5], who ascribed them to a Mo oxycarbide. However, while two of our reflectances gave d spacings close to those reported in [5], the third one was different (d=309 pm; [5]: 410 pm). We did not find a clear correlation between the intensity of these signals and the selectivity (cf. Fig. 3). While any relation between intensity of reflectances and quantity of a phase may be subject to many uncertainties (e.g. differ-
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ences in crystallinity, crystal size and shape), our results even do not sugges~t a correlation between the intensity of Mo oxx~rbide reflectances and selectivity. In the XPS y of reacted samples, no indication of a phase that might contain interstitial carbide was found. Since the isomerization site is tolerant towards handling in air, this is at variance with the oxycarbide being the isomerization site With transitional Mo oxides as starting materials, it was possible to decrease the length of the selectivity transient or avoid it at all (Fig. 5). While the decrease may be also due to accelerated reduction (increased density of structural defects), the immediate appearance of isomerization selectivity with triclinic Mo4OII suggests that the isomerization site is an oxide structure. Immediate appearance of isomerization selectivity was also observed by Matsuda et al. after mild reduction of MoO3 with H2 [12]. We propose, therefore, that the active site contains Mo ions in high oxidation states, possibly in a mixed-valence ensemble.
4. CONCLUSIONS Interaction of MoO3 with n-butane/H2 at 623 K leads to gradual development of a highly selective isomerization catalyst. The isomerization site survives contact with air under ambient conditions. XRD gives indications of oxycarbide formation, but no correlation with the isomerization selectivity. XPS provides no indication for the presence of oxycarbide after contact with air. High isomerization selectivity- without transient increase- is provided by starting with triclinic MO4Oll instead of MoO3. On this basis, the isomerization site is suggested to be an oxide structure with Mo in high oxidation state. ACKNOWLEDGEMENTS We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft. REFERENCES 1. J.S. Lee, S.T. Oyama, M. Boudart, J. Catal. 106 (1987), 125, 133. 2. F.H. Ribeiro, R.A. Dalla Betta, M. Boudart, et al., J. Catal. 130 (1992) 86. 3. M..J. Ledoux, C. Pham-Huu, H. Dunlop, J. Guille, Stud. Surf. Sci. Catal. 75 (1993) 955. 4. A. Frennet, G. Leclercq, L. Leclercq, et al., Stud. Surf. Sci. Catal. 75 (1993) 927. 5. P. Delporte, F. Meunier, C. Pham-Huu, M.J. Ledoux, et al., Catal. Today 23 (1995) 251. 6. P. DelGallo, F. Meunier, C. Pham-Huu, et al., Ind. Eng. Chem. Res. 36 (1997) 4166. 7. A. Katrib, P. Leflaive, L. Hilaire, G. Maire, Catal. Lett. 38 (1996) 95. 8. H. G. Boyen, "Macfit", available at the University of Basle, Institute of Physics. 9. B. Brox, I. Olefjord, Surf. Interface Anal. 13 (1988), 3. 10. W. Grtinert, A. Yu. Stakheev, R. Feldhaus, et al., J. Phys. Chem. 95 (1991), 1323. 11. M.J. Ledoux, F. Meunier, B. Heinrich, et al., Appl. Catal. A181 (1999), 157. 12. T. Matsuda, H. Shiro, H. Sakkagami, N. Takahashi, Catal. Lett. 47 (1997), 99.