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Modification of MoO3 surfaces by vapour-deposited cobalt atoms
Surface Science 433–435 (1999) 723–727 www.elsevier.nl/locate/susc
Modification of MoO surfaces by 3 vapour-deposited cobalt atoms R. Kleyna a, H. Mex a, M. Voß a, D. Borgmann a, *, L. Viscido b, J.M. Heras b a Institute of Physical and Theoretical Chemistry, University of Erlangen–Nu¨renberg, Egerlandstr. 3, D-91058 Erlangen, Germany b Institute of Physical Chemistry (INIFTA), University of La Plata, C.C. 16, Suc. 4, 1900 La Plata, Argentina
Abstract The interaction of evaporated cobalt atoms with polycrystalline MoO surfaces was studied by means of X-ray 3 photoelectron spectroscopy ( XPS). In several steps cobalt was evaporated onto the clean MoO surface at 300 K. 3 With increasing cobalt layer thickness a shoulder in the Mo 3d spectra indicated the formation of Mo4+ and Mo5+. From the very beginning of the evaporation the Co 2p spectra showed the appearance of Co2+ and at higher coverages that of Co0. Subsequent stepwise annealing of the sample up to 800 K revealed that nearly the whole amount of Co0 species is also oxidized and a non-stoichiometric cobalt molybdate is formed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Cobalt; Evaporation; Molybdenum oxides; Surface chemical reaction; X-ray photoelectron spectroscopy
1. Introduction Cobalt/molybdenum catalysts show versatile catalytic properties and are widely used in both selective oxidation and hydrogenation reactions. In the past surface oxidation reactions of cobalt and molybdenum have been studied by photoelectron spectroscopy and a number of papers have been published [1,2]. In the last decade our groups have investigated cobalt and molybdenum catalysts with respect to the oxygen-induced activation of polycrystalline cobalt surfaces [3], the oxidation of cobalt single crystals [4] and the adsorption of CO on polycrystalline molybdenum 2 [5]. The aim of this work is to study the interaction * Corresponding author. Fax: +49-9131-8528867. E-mail address: [email protected] (D. Borgmann)
of evaporated cobalt layers with an MoO /Mo 3 support.
2. Experimental The photoelectron spectroscopic studies were performed in an all-metal ultrahigh vacuum ( UHV ) system (ESCALAB 200, VG Scientific), equipped with facilities for electron spectroscopy [ X-ray photoelectron spectroscopy ( XPS), ultraviolet photoelectron spectroscopy ( UPS) and Auger electron spectroscopy (AES)], low-energy electron diffraction (LEED), a quadrupole mass spectrometer and a cobalt evaporation source. For the XPS studies, an excitation energy of 1486.6 eV (Al Ka) was applied. The MoO layers were produced on 3 thin molybdenum foils in a continuous-flow reactor at 800 K, 10 h and 1 atm of oxygen. Small
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contaminations of carbon could be removed by heating under UHV conditions. Previous attempts to clean the MoO surface by Ar+-ion bombard3 ment have shown that a reduction of MoO takes 3 place [6 ].
3. Results and discussion The Mo 3d XPS spectra taken after subsequent cobalt evaporation steps are given in Fig. 1. The topmost spectrum of MoO shows, besides the 3 signals of Mo6+ 3d (232.7 eV ) and Mo6+ 3d 5/2 3/2 (235.8 eV ), a shoulder and a small signal at the low-energy side of the spectrum. A Shirley background correction was applied to all spectra and the peaks were fitted using pseudo-Voigt lines. Additionally, a constant intensity ratio of the Mo 3d signals (and in the following paragraph 5/2,3/2 also of the Co 2p signals) and a constant 3/2,1/2 spin–orbit splitting of 3.1 eV were established. Applying the fit routine to the Mo 3d signals, the additional peaks are assigned to Mo5+ (231.1 eV )
and Mo4+ (229.6 eV ). A possible explanation for the appearance of molybdenum oxidation states lower than +6 on the freshly prepared MoO is its 3 reduction by small carbon contaminations with the formation of CO. A reduced interfacial oxide at the MoO /Mo interface can be ruled out, because 3 the MoO layer thickness amounts to more than 3 20 nm as has been checked by separate X-ray diffraction experiments. Under these conditions the interface is not visible to XPS. As can be seen from Table 1, the XPS binding energies of Mo6+ and Mo5+ obtained correlate very well with data from the literature. The Mo4+ signal shows small deviations of 0.1 eV [7] and 0.4 eV [2], respectively. The accuracy of the fitted binding energies is ±0.1 eV. Cobalt was evaporated in six steps onto the clean MoO surface at 300 K (130 min total evapo3 ration time). A total cobalt layer thickness of ca. five monolayers can be estimated from the attenuation of the Mo 3d signal intensity. Due to the strong interaction between cobalt and MoO , lat3 eral diffusion seems unlikely and a layer-by-layer growth mechanism is assumed. With increasing cobalt layer thickness a shoulder in the Mo 3d spectra grows, indicating the formation of Mo5+ and Mo4+. The relative intensities of the molybdenum signals obtained by the fit routines as a function of the cobalt evaporation time are plotted in Fig. 2. The increase of the intensities of Mo5+ and Mo4+ at the cost of the Mo6+ intensity can clearly be seen. Fig. 3 shows the Co 2p spectra taken after different evaporation times up to 130 min. From Table 1 XPS binding energies (in eV ) of the Mo 3d and Co 2p 5/2 3/2 core levels
Fig. 1. Mo 3d XPS spectra taken at 300 K after subsequent evaporation of cobalt.
Mo6+ (MoO ) 3 Mo6+ (CoMoO ) 4 Mo5+ Mo4+ (MoO ) 2 Co0 Co2+ (CoO) Co2+ (Mo matrix) Co2+ (CoMoO ) 4
This work
Literature data
232.7 232.3 231.1 229.6 778.4 780.7 781.1 781.2
232.7 232.2 231.1 229.5
[7], 232.7 [2] [2] [2] [7], 229.2 [2]
780.0 [8], 780.1 [2] 780.8 [2]
R. Kleyna et al. / Surface Science 433–435 (1999) 723–727
Fig. 2. Relative intensities of Mo4+, Mo5+ and Mo6+ as a function of evaporation time based on the data in Fig. 1.
725
the appearance of shake-up satellites at the highenergy side of the spectra [9]. The shoulder at the low-energy side (773.8 eV ) is assigned to the Co L M M Auger signal. The small shift of 3 45 45 0.4 eV to higher binding energies at the beginning of the evaporation is attributed to an influence of the molybdenum matrix on the Co2+. Comparison with reference data taken with the same spectrometer and with the literature [9] shows that at the end of the deposition the Co2+ is still influenced by the molybdenum matrix, since there are deviations in the binding energy of 0.7 eV [8] and 0.6 eV [2], respectively, as compared with CoO (see also Table 1). With increasing evaporation time the Co0 signal at 778.4 eV can also be observed in the Co 2p spectrum. After finishing the deposition procedure, the layer system was heated in steps of 100 K up to 800 K, at which temperature an annealing time of 2 h was applied. Figs. 4 and 5 demonstrate the changes in the Mo 3d and Co 2p spectra, respectively. Strong changes in the Mo 3d spectra occur at temperatures above 500 K: an Mo6+ 3d feature
Fig. 3. Co 2p XPS spectra taken at 300 K after subsequent evaporation of cobalt.
the very beginning of the evaporation the Co 2p spectra reveal the appearance of a small amount of Co2+ at a binding energy of 781.1 eV. With increasing cobalt layer thickness an increase in intensity and a shift of the Co2+ signal to 780.7 eV are observed. Typical of the formation of Co2+ is
Fig. 4. Mo 3d XPS spectra taken at different temperatures after evaporation of cobalt.
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R. Kleyna et al. / Surface Science 433–435 (1999) 723–727
0.4 eV to lower binding energies above 500 K can only be explained by the formation of a cobalt molybdate. McIntyre et al. [2] report an Mo6+ 3d binding energy of 232.2 eV taken at CoMoO , which correlates very well with our own 4 value of 232.3 eV. The appearance of Mo4+ leads to the assumption that a non-stoichiometric cobalt molybdate, CoMoO , with an oxygen deficiency, 4−x is formed on the support layer.
4. Summary
Fig. 5. Co 2p XPS spectra taken at different temperatures after evaporation of cobalt.
is obviously reestablished; however, there is a shift of 0.4 eV towards lower binding energy compared with MoO (Fig. 4). The fit procedure applied to 3 the Mo 3d XP spectra yields a decrease of the signal intensities of Mo4+ and Mo( VI )O , a disap3 pearance of the Mo5+ peak and an increase of the peak intensity of Mo6+ (with a shift of 0.4 eV ). In the same way, the Co 2p spectra in Fig. 5 show a characteristic change from a Co0 intensity at 300 K (topmost spectrum) to a strong Co2+ intensity at 800 K. A shift of the Co2+ intensity from 780.7 to 781.2 eV indicates a change in the composition of the surface layer (see below). The Mo 3d and Co 2p spectra point to the appearance of an interdiffusion process and following redox reactions between the molybdenum and cobalt layers. Angle-resolved XPS was performed in order to characterize the interdiffusion processes. Unfortunately, there was no significant difference in the XPS spectra taken at take-off angles of 0° and 50° with respect to the surface normal, owing to the high roughness of the MoO layer. The shift of the Mo6+ signal by 3
In the present study the interaction of thin evaporated cobalt layers with an MoO /Mo sup3 port was investigated. During the evaporation procedure at 300 K, Mo6+ is partially reduced to Mo5+ and Mo4+. The deposited cobalt layer reacts at once with MoO with the formation of Co2+. 3 With increasing cobalt layer thickness Co0 becomes detectable on the topmost surface layer. Annealing of the layer system to 800 K leads to a shift in the Mo6+ 3d signal of 0.4 eV to lower binding energy, a decrease of the Mo4+ intensity and disappearance of the Mo5+ peak. Interdiffusion processes accelerate an oxidation of nearly all of the Co0 species to Co2+ and a shift of the Co2+ signal can be observed. The comparison with literature data leads to the conclusion that after the annealing procedure, a non-stoichiometric cobalt molybdate exists on the surface.
Acknowledgements The authors thank the BMBF (ARG 6L1A6A), the Verband der Chemischen Industrie, SeCyt (Project 5.152-8/96) and CONICET (Argentina) for financial support. Fruitful discussions with Professor Steinru¨ck and Professor Wedler are appreciated.
References [1] I. Alstrup, I. Chorkendorff, R. Candia, B.S. Clausen, H. Topso¨e, J. Catal. 77 (1982) 397. [2] N.S. McIntyre, D.D. Johnston, L.L. Coatsworth, R.D. Davidson, J.R. Brown, Surf. Interface Anal. 15 (1990) 265 and references therein.
R. Kleyna et al. / Surface Science 433–435 (1999) 723–727 [3] G. Fro¨hlich, U. Kestel, J. Łojewska, T. Łojewski, G. Meyer, M. Voß, D. Borgmann, R. Dziembaj, G. Wedler, Appl. Catal. A: General 134 (1996) 1. [4] B. Klingenberg, F. Grellner, D. Borgmann, G. Wedler, Surf. Sci. 296 (1993) 374. [5] L.D. Lo´pez-Carren˜o, J.M. Heras, L. Viscido, Surf. Sci. 377–379 (1997) 615.
727
[6 ] L.D. Lo´pez-Carren˜o, G. Benı´tez, L. Viscido, J.M. Heras, F. Yubero, J.P. Espino´s, A.R. Gonza´lez-Elipe, Surf. Interface Anal. 26 (1998) 235. [7] B. Brox, I. Olefjord, Surf. Interface Anal. 13 (1988) 3. [8] K.S. Kim, Phys. Rev. B 11 (1975) 2177. [9] V.M. Jime´nez, A. Ferna´ndez, J.P. Espino´s, A.R. Gonza´lezElipe, J. Electron Spectrosc. Relat. Phenom. 71 (1995) 61.
Modification of MoO surfaces by 3 vapour-deposited cobalt atoms R. Kleyna a, H. Mex a, M. Voß a, D. Borgmann a, *, L. Viscido b, J.M. Heras b a Institute of Physical and Theoretical Chemistry, University of Erlangen–Nu¨renberg, Egerlandstr. 3, D-91058 Erlangen, Germany b Institute of Physical Chemistry (INIFTA), University of La Plata, C.C. 16, Suc. 4, 1900 La Plata, Argentina
Abstract The interaction of evaporated cobalt atoms with polycrystalline MoO surfaces was studied by means of X-ray 3 photoelectron spectroscopy ( XPS). In several steps cobalt was evaporated onto the clean MoO surface at 300 K. 3 With increasing cobalt layer thickness a shoulder in the Mo 3d spectra indicated the formation of Mo4+ and Mo5+. From the very beginning of the evaporation the Co 2p spectra showed the appearance of Co2+ and at higher coverages that of Co0. Subsequent stepwise annealing of the sample up to 800 K revealed that nearly the whole amount of Co0 species is also oxidized and a non-stoichiometric cobalt molybdate is formed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Cobalt; Evaporation; Molybdenum oxides; Surface chemical reaction; X-ray photoelectron spectroscopy
1. Introduction Cobalt/molybdenum catalysts show versatile catalytic properties and are widely used in both selective oxidation and hydrogenation reactions. In the past surface oxidation reactions of cobalt and molybdenum have been studied by photoelectron spectroscopy and a number of papers have been published [1,2]. In the last decade our groups have investigated cobalt and molybdenum catalysts with respect to the oxygen-induced activation of polycrystalline cobalt surfaces [3], the oxidation of cobalt single crystals [4] and the adsorption of CO on polycrystalline molybdenum 2 [5]. The aim of this work is to study the interaction * Corresponding author. Fax: +49-9131-8528867. E-mail address: [email protected] (D. Borgmann)
of evaporated cobalt layers with an MoO /Mo 3 support.
2. Experimental The photoelectron spectroscopic studies were performed in an all-metal ultrahigh vacuum ( UHV ) system (ESCALAB 200, VG Scientific), equipped with facilities for electron spectroscopy [ X-ray photoelectron spectroscopy ( XPS), ultraviolet photoelectron spectroscopy ( UPS) and Auger electron spectroscopy (AES)], low-energy electron diffraction (LEED), a quadrupole mass spectrometer and a cobalt evaporation source. For the XPS studies, an excitation energy of 1486.6 eV (Al Ka) was applied. The MoO layers were produced on 3 thin molybdenum foils in a continuous-flow reactor at 800 K, 10 h and 1 atm of oxygen. Small
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 01 5 8 -2
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R. Kleyna et al. / Surface Science 433–435 (1999) 723–727
contaminations of carbon could be removed by heating under UHV conditions. Previous attempts to clean the MoO surface by Ar+-ion bombard3 ment have shown that a reduction of MoO takes 3 place [6 ].
3. Results and discussion The Mo 3d XPS spectra taken after subsequent cobalt evaporation steps are given in Fig. 1. The topmost spectrum of MoO shows, besides the 3 signals of Mo6+ 3d (232.7 eV ) and Mo6+ 3d 5/2 3/2 (235.8 eV ), a shoulder and a small signal at the low-energy side of the spectrum. A Shirley background correction was applied to all spectra and the peaks were fitted using pseudo-Voigt lines. Additionally, a constant intensity ratio of the Mo 3d signals (and in the following paragraph 5/2,3/2 also of the Co 2p signals) and a constant 3/2,1/2 spin–orbit splitting of 3.1 eV were established. Applying the fit routine to the Mo 3d signals, the additional peaks are assigned to Mo5+ (231.1 eV )
and Mo4+ (229.6 eV ). A possible explanation for the appearance of molybdenum oxidation states lower than +6 on the freshly prepared MoO is its 3 reduction by small carbon contaminations with the formation of CO. A reduced interfacial oxide at the MoO /Mo interface can be ruled out, because 3 the MoO layer thickness amounts to more than 3 20 nm as has been checked by separate X-ray diffraction experiments. Under these conditions the interface is not visible to XPS. As can be seen from Table 1, the XPS binding energies of Mo6+ and Mo5+ obtained correlate very well with data from the literature. The Mo4+ signal shows small deviations of 0.1 eV [7] and 0.4 eV [2], respectively. The accuracy of the fitted binding energies is ±0.1 eV. Cobalt was evaporated in six steps onto the clean MoO surface at 300 K (130 min total evapo3 ration time). A total cobalt layer thickness of ca. five monolayers can be estimated from the attenuation of the Mo 3d signal intensity. Due to the strong interaction between cobalt and MoO , lat3 eral diffusion seems unlikely and a layer-by-layer growth mechanism is assumed. With increasing cobalt layer thickness a shoulder in the Mo 3d spectra grows, indicating the formation of Mo5+ and Mo4+. The relative intensities of the molybdenum signals obtained by the fit routines as a function of the cobalt evaporation time are plotted in Fig. 2. The increase of the intensities of Mo5+ and Mo4+ at the cost of the Mo6+ intensity can clearly be seen. Fig. 3 shows the Co 2p spectra taken after different evaporation times up to 130 min. From Table 1 XPS binding energies (in eV ) of the Mo 3d and Co 2p 5/2 3/2 core levels
Fig. 1. Mo 3d XPS spectra taken at 300 K after subsequent evaporation of cobalt.
Mo6+ (MoO ) 3 Mo6+ (CoMoO ) 4 Mo5+ Mo4+ (MoO ) 2 Co0 Co2+ (CoO) Co2+ (Mo matrix) Co2+ (CoMoO ) 4
This work
Literature data
232.7 232.3 231.1 229.6 778.4 780.7 781.1 781.2
232.7 232.2 231.1 229.5
[7], 232.7 [2] [2] [2] [7], 229.2 [2]
780.0 [8], 780.1 [2] 780.8 [2]
R. Kleyna et al. / Surface Science 433–435 (1999) 723–727
Fig. 2. Relative intensities of Mo4+, Mo5+ and Mo6+ as a function of evaporation time based on the data in Fig. 1.
725
the appearance of shake-up satellites at the highenergy side of the spectra [9]. The shoulder at the low-energy side (773.8 eV ) is assigned to the Co L M M Auger signal. The small shift of 3 45 45 0.4 eV to higher binding energies at the beginning of the evaporation is attributed to an influence of the molybdenum matrix on the Co2+. Comparison with reference data taken with the same spectrometer and with the literature [9] shows that at the end of the deposition the Co2+ is still influenced by the molybdenum matrix, since there are deviations in the binding energy of 0.7 eV [8] and 0.6 eV [2], respectively, as compared with CoO (see also Table 1). With increasing evaporation time the Co0 signal at 778.4 eV can also be observed in the Co 2p spectrum. After finishing the deposition procedure, the layer system was heated in steps of 100 K up to 800 K, at which temperature an annealing time of 2 h was applied. Figs. 4 and 5 demonstrate the changes in the Mo 3d and Co 2p spectra, respectively. Strong changes in the Mo 3d spectra occur at temperatures above 500 K: an Mo6+ 3d feature
Fig. 3. Co 2p XPS spectra taken at 300 K after subsequent evaporation of cobalt.
the very beginning of the evaporation the Co 2p spectra reveal the appearance of a small amount of Co2+ at a binding energy of 781.1 eV. With increasing cobalt layer thickness an increase in intensity and a shift of the Co2+ signal to 780.7 eV are observed. Typical of the formation of Co2+ is
Fig. 4. Mo 3d XPS spectra taken at different temperatures after evaporation of cobalt.
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R. Kleyna et al. / Surface Science 433–435 (1999) 723–727
0.4 eV to lower binding energies above 500 K can only be explained by the formation of a cobalt molybdate. McIntyre et al. [2] report an Mo6+ 3d binding energy of 232.2 eV taken at CoMoO , which correlates very well with our own 4 value of 232.3 eV. The appearance of Mo4+ leads to the assumption that a non-stoichiometric cobalt molybdate, CoMoO , with an oxygen deficiency, 4−x is formed on the support layer.
4. Summary
Fig. 5. Co 2p XPS spectra taken at different temperatures after evaporation of cobalt.
is obviously reestablished; however, there is a shift of 0.4 eV towards lower binding energy compared with MoO (Fig. 4). The fit procedure applied to 3 the Mo 3d XP spectra yields a decrease of the signal intensities of Mo4+ and Mo( VI )O , a disap3 pearance of the Mo5+ peak and an increase of the peak intensity of Mo6+ (with a shift of 0.4 eV ). In the same way, the Co 2p spectra in Fig. 5 show a characteristic change from a Co0 intensity at 300 K (topmost spectrum) to a strong Co2+ intensity at 800 K. A shift of the Co2+ intensity from 780.7 to 781.2 eV indicates a change in the composition of the surface layer (see below). The Mo 3d and Co 2p spectra point to the appearance of an interdiffusion process and following redox reactions between the molybdenum and cobalt layers. Angle-resolved XPS was performed in order to characterize the interdiffusion processes. Unfortunately, there was no significant difference in the XPS spectra taken at take-off angles of 0° and 50° with respect to the surface normal, owing to the high roughness of the MoO layer. The shift of the Mo6+ signal by 3
In the present study the interaction of thin evaporated cobalt layers with an MoO /Mo sup3 port was investigated. During the evaporation procedure at 300 K, Mo6+ is partially reduced to Mo5+ and Mo4+. The deposited cobalt layer reacts at once with MoO with the formation of Co2+. 3 With increasing cobalt layer thickness Co0 becomes detectable on the topmost surface layer. Annealing of the layer system to 800 K leads to a shift in the Mo6+ 3d signal of 0.4 eV to lower binding energy, a decrease of the Mo4+ intensity and disappearance of the Mo5+ peak. Interdiffusion processes accelerate an oxidation of nearly all of the Co0 species to Co2+ and a shift of the Co2+ signal can be observed. The comparison with literature data leads to the conclusion that after the annealing procedure, a non-stoichiometric cobalt molybdate exists on the surface.
Acknowledgements The authors thank the BMBF (ARG 6L1A6A), the Verband der Chemischen Industrie, SeCyt (Project 5.152-8/96) and CONICET (Argentina) for financial support. Fruitful discussions with Professor Steinru¨ck and Professor Wedler are appreciated.
References [1] I. Alstrup, I. Chorkendorff, R. Candia, B.S. Clausen, H. Topso¨e, J. Catal. 77 (1982) 397. [2] N.S. McIntyre, D.D. Johnston, L.L. Coatsworth, R.D. Davidson, J.R. Brown, Surf. Interface Anal. 15 (1990) 265 and references therein.
R. Kleyna et al. / Surface Science 433–435 (1999) 723–727 [3] G. Fro¨hlich, U. Kestel, J. Łojewska, T. Łojewski, G. Meyer, M. Voß, D. Borgmann, R. Dziembaj, G. Wedler, Appl. Catal. A: General 134 (1996) 1. [4] B. Klingenberg, F. Grellner, D. Borgmann, G. Wedler, Surf. Sci. 296 (1993) 374. [5] L.D. Lo´pez-Carren˜o, J.M. Heras, L. Viscido, Surf. Sci. 377–379 (1997) 615.
727
[6 ] L.D. Lo´pez-Carren˜o, G. Benı´tez, L. Viscido, J.M. Heras, F. Yubero, J.P. Espino´s, A.R. Gonza´lez-Elipe, Surf. Interface Anal. 26 (1998) 235. [7] B. Brox, I. Olefjord, Surf. Interface Anal. 13 (1988) 3. [8] K.S. Kim, Phys. Rev. B 11 (1975) 2177. [9] V.M. Jime´nez, A. Ferna´ndez, J.P. Espino´s, A.R. Gonza´lezElipe, J. Electron Spectrosc. Relat. Phenom. 71 (1995) 61.