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XPS and TPD study of CO interaction with Pd–Al and Pd–Al2O3 systems
Journal of Electron Spectroscopy and Related Phenomena 114–116 (2001) 327–332 www.elsevier.nl / locate / elspec
XPS and TPD study of CO interaction with Pd–Al and Pd–Al 2 O 3 systems ´ *, V. Johanek, ´ ´ N. Tsud, K. Veltruska´ V. Matolın I. Stara, ˇ ´ 2, 18000 Prague 8, Czech Republic Department of Electronics and Vacuum Physics, Charles University, V Holesovickach Received 8 August 2000; received in revised form 26 September 2000; accepted 3 October 2000
Abstract The metal–substrate and metal–metal interactions represent important effects determining the properties of supported catalysts. We investigate the CO adsorption mechanism on Pd particles supported on Al 2 O 3 and Al by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). CO–metal interaction is investigated by TP desorption of CO and by XPS of C 1s intensities that exhibit three CO-related components. The dissociation activity is monitored as a rise of C 1s signal at 285 eV while the molecularly adsorbed CO exhibits the intensity at 287 eV. Pd–substrate interaction is determined from the Pd core- and valence-level binding energy variations. The influence of the Pd–Al alloying process on the Pd MNN Auger transition is shown. The studies show that the metal–substrate and metal–metal interactions play an important role in CO–Pd adsorption process. 2001 Elsevier Science B.V. All rights reserved. Keywords: Pd; Al; Al 2 O 3 ; XPS; Metal–substrate interaction; CO adsorption; Bimetallic interaction
1. Introduction The adsorption and reaction studies are often performed on so called model catalysts — welldefined systems, such as single crystals and supported ultra-thin metallic layers, characterized by surface-science techniques. Adsorption and reaction on small supported particles, which represent more reliable models of real catalysts, often differ strongly from the behavior of macroscopic metal surfaces. The interaction of CO with highly dispersed platinum metals on non-metallic supports is of *Corresponding author. Tel.: 1420-2-2191-2323; fax: 1420-26885-095. ´ E-mail address: [email protected] (V. Matolın).
special interest, because CO is involved in many practical reactions and its relatively simple and wellknown molecular structure enables it to be used as a probe for adsorption mechanism studies. Many studies have been performed on Pd model catalysts, supported by Al 2 O 3 and MgO, due to the great importance of Pd in heterogeneous catalysis [1–8]. In the case of supported catalysts, the metal– substrate interaction (MSI), can influence the adsorption and morphological size-dependent properties of particles. While alumina is considered as an inert, non-reactive substrate [9] by some authors, others classify it as more or less reactive [10,11]. Investigation of size effect upon CO adsorption on alumina supported particles showed a surprising effect – CO partially dissociated on small Pd clusters
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00335-2
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´ et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 327 – 332 V. Matolın
deposited on mica [12], g-alumina [1] and thin aluminum oxide film [13]. On the other hand, CO adsorption was molecular on Pd clusters deposited on sapphire [5,14] and Al 2 O 3 (111) / NiAl(110) [2]. The possible origin of this phenomenon is the metal– substrate interaction (MSI), which could be related to the MS charge transfer. MSI-induced changes in the Pd–CO interaction are much more important in the case of Pd films deposited on metallic supports. The combined X-ray photoelectron (XPS), ultra-violet photoelectron spectroscopy (UPS) and CO-TPD studies have been performed on Pd overlayers on transition metals Ta [15], Mo [16], W, Re, Ru [17], Rh, Pt [17,18], noble metals, such as Au [19], and Al [20–29]. The common feature of Pd deposited on Al and transition metals was the shift of Pd core levels and the center of Pd d-band toward higher binding energies, due to the formation of a bimetallic bond. Originally, the deep-lying Pd 4d-states of Pd–Al were explained in terms of weak Pd–Al interaction giving atomic-like 4d 10 5s 0 electronic configuration [20,21,23,24]. In a second, more recent model the valence and core level BE shift has been attributed, in contrary, to a strong MS interaction [22,26] as a consequence of d–s,p rehybridization and d-electron charge transfer toward the Al substrate [27–29]. For the metal–CO interaction it was shown that the d-band centroid shift, away from the 2p* CO orbitals, leaded to a weakening of the Pd 4d–CO 2p* bonding and to the decrease of CO desorption temperature [30]. In order to explain a possible bimetallic Pd–Al effect, we compare the properties of Pd /Al 2 O 3 , Pd / Al and bulk Pd. XPS and ISS results show a strong interaction between Pd and Al leading to the formation of PdAl alloy of noble metal-like electronic structure. The CO adsorption–desorption studies on Pd /Al systems show an important decrease in the CO desorption temperature accompanied by partial CO dissociation.
2. Experimental XPS and ISS experiments were performed using the Omicron EA 125 multichannel hemispherical analyzer and a dual Al / Mg X-ray source. The substrate cleanliness, stoichiometry and amount of
deposited Pd were checked by XPS. The deposited layer morphology on alumina was investigated using QUASESE software (the software package for quantitative XPS of surface nanostructures by analysis of the peak shape and background). The principles of the method were explained in [31,32]. TPD experiment was carried out using the quadrupole mass spectrometer with a programmable sample heating in the 100–800 K range. The (0001) a-alumina substrate was prepared by heating in the air (at 1620 K for 2 h) and by argon ion bombardment (500 eV, 1 mA cm 22 , 5 min). The Al- and Pd-poly-crystalline samples were cleaned by cycles of argon ion bombardment (900 eV, 2 mA cm 22 ) and heating in vacuum at 900 K. The Pd thin film, equivalent to 1 ML, were deposited on the samples in situ at room temperature using the micro electron beam evaporation source (MEBES) [33] which enables the evaporation rate to be controlled by monitoring the Pd 1 ion current.
3. Results
3.1. Pd–substrate interaction study In order to elucidate the difference between the Pd /Al and Pd /Al 2 O 3 systems, we deposited the equivalent of 1 ML of Pd, at room temperature, on the air annealed (0001)a-Al 2 O 3 and on the argon ion sputtered Al substrates. Using the Pd 3d 5 / 2 peak and background form analysis by the QUASES software, it has been found that Pd formed three-dimensional islands on the alumina substrate. The Pd growth mode on the Al substrate was different, which can be seen in Fig. 1, where the results of the ion scattering spectroscopy (ISS), performed using the 2 keV He 1 ion beam, are shown. The Pd /Al intensity ratios, directly indicating the relative Pd /Al coverage, were 1.7 and 0.15, respectively, which confirmed the earlier observations of Pd–Al alloy formation accompanied by the Pd sub-surface diffusion [22,26]. The XPS studies, obtained by using Mg Ka radiation, are shown in Figs. 2 and 3, where the valence and core Pd3d levels, and the MNN Auger peaks are presented. For comparison the same features for bulk Pd can be seen in Fig. 4. The valence
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Fig. 1. Comparison of the ISS spectra for Pd / aluminum and Pd / a-Al 2 O 3 systems.
Fig. 2. XPS spectra of the a-Al 2 O 3 supported Pd. Top panel: Auger transitions M 4( 5 ) N 45 N 45 and 3d 3 / 2 core level lines of Pd. Lower panel: valence band.
band spectrum in Fig. 2 shows the formation of the Pd bulk-like d-band close to the zero BE (see Fig. 4), that overlaps the alumina valence band. Similarly the Pd3d peak energies for alumina supported particles (Fig. 2) are close to the bulk values (Fig. 4). The small difference between the ‘particle’ and ‘bulk’ BE is given by the general trend of BE variations, which is high for the small clusters and decreases to the bulk value as the number of cluster atoms increases. It is given by the size-dependent reduction of screening effects and final-state energy variation, due to particle charging in the case of poorly conductive substrate [34–36]. The effects of Pd /Al interaction can be seen in Fig. 3: the 3.1 eV shift of the Pd 4d-centroid (comparing to the 2.7 eV shift reported in [28,29] and 2.5 eV in [24] for Al (111)), and the 2 eV shift of the
Pd 3d peak (1.8 eV in [28,29]) to higher BE, relative to the bulk values, due to the strong hybridization of the Pd-d and Al-s states. In Figs. 2–4 it can be seen that also the Pd MNN Auger transition depends on Pd–substrate interaction. It can be decomposed into two elemental, 5 eV different, M 5 N 4,5 N 4,5 and M 4 N 4,5 N 4,5 peaks. The Auger kinetic energy, related to the Fermi level, can be roughly calculated using the first approximation formula, KE(MNN)5BE(M)2BE(N)2BE(N). The difference of several eV between the calculated and measured KE is due to the neglect of the hole–hole interaction energy and the two-hole relaxation energy, which should be considered in the exact calculations. However, this simple approach explains well the relationship, which can be deduced from Figs. 3 and 4, for the Pd Auger and Pd 4d-band
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Fig. 3. XPS spectra of the Pd /Al alloy. Top panel: Auger transitions M 4( 5 ) N 45 N 45 and 3d 3 / 2 core level lines of Pd. Lower panel: valence band.
energy shift DKE(MNN)52DBE(N), because both M and N levels move in the similar manner giving D[BE(M)2BE(N)]50. The second effect, characteristic for the Pd–Al alloy formation, is the separation of M 4 - and M 5 -related components of the MNN Auger transition. Figs. 2–4 show that this behavior is directly linked to the width of the involved (M) and (N) peaks. The narrower the alloy peaks are, the more the MNN Auger peak is structured.
3.2. CO adsorption In order to understand the influence of the Pd–Al interaction on CO adsorption, we investigated CO adsorption–desorption process on Pd /Al by means of TPD and XPS. For comparison, we also performed
Fig. 4. XPS spectra of Pd bulk. Top panel: Auger transitions M 4( 5 ) N 45 N 45 and 3d 3 / 2 core level lines of Pd. Lower panel: valence band.
CO desorption studies, for the same exposures, on a bare Al substrate. TPD results showed that CO desorption temperature decreased substantially for the Pd–Al alloy. Since the CO TPD features for the bulk Pd and Pd / a-Al 2 O 3 exhibited maximum at 475 K [1,5,8], the Pd–Al alloy possessed the maximum at 350 K. This variation of CO adsorption properties can be explained by the formation of a noble metal-like alloy characterized by a lower-lying d-band and a low density of states at the Fermi level, which weakens the Pd 4d–CO 2p* bonding interactions. While it was well established that CO adsorbs molecularly on the bulk and a-Al 2 O 3 supported Pd
´ et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 327 – 332 V. Matolın
[1,5], the XPS studies of the CO adsorption on the Pd /Al alloy showed a different behavior. The interaction of CO with Pd was determined from 1s line of carbon in photoelectron spectra that exhibited three CO-related components. The production of an atomic-like carbon was monitored as a rise of the C 1s signal (with peak at 285 eV) while the molecularly adsorbed CO exhibited the intensity at BE of 287 eV. The spectra exhibited also a peak appearing at 282 eV, which was associated with a metal–carbide species. Fig. 5 illustrates the result of the decomposition of the C 1s-related peaks as a function of the CO adsorption / desorption cycles. Two exposures of 10 Langmuirs (10310 26 Torr s) at room temperature were followed by the desorption heating up to 550 K. The periodic ‘CO peak’ intensity variations, related to the intensity of Pd3d 5 / 2 peak, corresponded to concentration variations of the adsorbed CO molecules during the adsorption–desorption cycles. The most important result of this study was the continuous increase in the atomic-carbon intensity, indicating a partial CO dissociation within both adsorption and desorption processes. Since the ‘reference’ experiment on a clean Al substrate, performed before the Pd deposition at the same conditions, did not show any carbon intensity increase, we can conclude that Pd, interacting with aluminum, exhibited a partial CO dissociation, which accompanied the decrease in the CO-desorption activation energy. Fig. 5 also shows, that the deposited carbon
331
is partially transformed, during the cycles, to the metal–carbide form. One can conclude that the Pd d-Al s,p charge transfer, which is accompanied by the Pd–Al work function decrease [28,29], can enhance the 2p* CO antibonding orbital filling by delocalized s,p electrons leading to the weakening of the C–O bond.
4. Summary The Pd / alumina and Pd /Al 2 O 3 interaction can be well characterized by means of XPS. Pd on the a-Al 2 O 3 substrate exhibits the Pd-bulk-like XPS features indicating a weak MS interaction. This is in agreement with the well-established statement that a-Al 2 O 3 is a relatively inert catalyst support. On the other hand Pd is strongly interacting with metallic Al, forming Pd–Al surface alloy, for which the Pd–Al intermixing and a strong electronic interaction are characteristic. The alloy formation is accompanied by the increase of the core- and valence-level BE. The significant narrowing of the valence d-band leads to the narrowing and structuring of the Pd MNN Auger peak, which moves towards lower KE. The Pd–Al alloying decreases simultaneously the CO desorption and dissociation energy.
Acknowledgements This work was supported by the Czech Grant Agency under the Grants No. 202 / 99 / 1714, and by the Czech Ministry of Education, Youth and Sports under the Project No. VS97116.
References
Fig. 5. Variations of XPS relative intensities of C 1s peak components with CO adsorption–desorption cycles.
´ V. Matolın, ´ Surf. Sci. 313 (1994) 99. [1] I. Stara, [2] M. Baumer, J. Libuda, A. Sandell, H.J. Freund, G. Graw, Th. Bertrams, H. Neddermeyer, Ber. Bunsenges, Phys. Chem. 99 (1995) 1381. [3] C.R. Henry, C. Chapon, C. Duriez, Z. Phys. D: Atoms, Molecules and Clusters 19 (1991) 347. ´ V. Nehasil, V. Matolın, ´ Surf. Sci. 331–333 (1995) [4] I. Stara, 173. ´ V. Matolın, ´ Surf. Rev. Lett. 4 (1997) 1353. [5] I. Stara, [6] A. Sandell, J. Libuda, P.A. Bruhwiler, S. Andersson, A.J.
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[21] L.Q. Jiang, M.W. Ruckman, M. Strongin, Phys. Rev. B 39 (1989) 1564. [22] V. Shutthanandan, A.A. Saleh, N.R. Shivaparan, R.J. Smith, Surf. Sci. 350 (1996) 11. [23] R.J. Smith, A.W. Dernier van der Gon, J.F. van der Veen, Phys. Rev. B 38 (1988) 12712. [24] B. Frick, K. Jacobi, Surf. Sci. 178 (1986) 907. [25] B. Frick, K. Jacobi, J.A. Wilder, H.J. Sagner, K.H. Frank, Surf. Sci. 193 (1988) 529. [26] R.J. Smith, A.W. Dernier van der Gon, J.F. van der Veen, Surf. Sci 233 (1990) 103. [27] Y. Jeon, J. Chen, M. Croft, Phys. Rev. B 50 (1994) 6555. [28] J.A. Rodriguez, Surf. Sci. 318 (1994) 253. [29] J.A. Rodriguez, Surf. Sci. 345 (1996) 347. [30] B. Hammer, Y. Morikawa, J.K. Nørskov, Phys. Rev. Lett. 76 (1996) 2141. [31] S. Tougaard, J. Vac. Sci. Technol. A 14 (3) (1996) 1415. [32] Quantitative Analysis of Surfaces by Electron Spectroscopy — User’s Guide, QUASES Tougaard Aps, Odense, 1999. ´ K. Masek, ˇ [33] V. Matolın, O. Moreau, V. Nehasil, Czech. J. Phys. 47 (1997) 261. [34] G.K. Wertheim, Z. Phys. D 12 (1989) 319. [35] G.K. Wertheim, Phys. Rev. B 36 (1987) 9559. [36] G.K. Wertheim, S.B. DiCenzo, D.N.E. Buchanan, P.A. Bennet, Sol. State Commun. 53 (1985) 377.
XPS and TPD study of CO interaction with Pd–Al and Pd–Al 2 O 3 systems ´ *, V. Johanek, ´ ´ N. Tsud, K. Veltruska´ V. Matolın I. Stara, ˇ ´ 2, 18000 Prague 8, Czech Republic Department of Electronics and Vacuum Physics, Charles University, V Holesovickach Received 8 August 2000; received in revised form 26 September 2000; accepted 3 October 2000
Abstract The metal–substrate and metal–metal interactions represent important effects determining the properties of supported catalysts. We investigate the CO adsorption mechanism on Pd particles supported on Al 2 O 3 and Al by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). CO–metal interaction is investigated by TP desorption of CO and by XPS of C 1s intensities that exhibit three CO-related components. The dissociation activity is monitored as a rise of C 1s signal at 285 eV while the molecularly adsorbed CO exhibits the intensity at 287 eV. Pd–substrate interaction is determined from the Pd core- and valence-level binding energy variations. The influence of the Pd–Al alloying process on the Pd MNN Auger transition is shown. The studies show that the metal–substrate and metal–metal interactions play an important role in CO–Pd adsorption process. 2001 Elsevier Science B.V. All rights reserved. Keywords: Pd; Al; Al 2 O 3 ; XPS; Metal–substrate interaction; CO adsorption; Bimetallic interaction
1. Introduction The adsorption and reaction studies are often performed on so called model catalysts — welldefined systems, such as single crystals and supported ultra-thin metallic layers, characterized by surface-science techniques. Adsorption and reaction on small supported particles, which represent more reliable models of real catalysts, often differ strongly from the behavior of macroscopic metal surfaces. The interaction of CO with highly dispersed platinum metals on non-metallic supports is of *Corresponding author. Tel.: 1420-2-2191-2323; fax: 1420-26885-095. ´ E-mail address: [email protected] (V. Matolın).
special interest, because CO is involved in many practical reactions and its relatively simple and wellknown molecular structure enables it to be used as a probe for adsorption mechanism studies. Many studies have been performed on Pd model catalysts, supported by Al 2 O 3 and MgO, due to the great importance of Pd in heterogeneous catalysis [1–8]. In the case of supported catalysts, the metal– substrate interaction (MSI), can influence the adsorption and morphological size-dependent properties of particles. While alumina is considered as an inert, non-reactive substrate [9] by some authors, others classify it as more or less reactive [10,11]. Investigation of size effect upon CO adsorption on alumina supported particles showed a surprising effect – CO partially dissociated on small Pd clusters
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00335-2
328
´ et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 327 – 332 V. Matolın
deposited on mica [12], g-alumina [1] and thin aluminum oxide film [13]. On the other hand, CO adsorption was molecular on Pd clusters deposited on sapphire [5,14] and Al 2 O 3 (111) / NiAl(110) [2]. The possible origin of this phenomenon is the metal– substrate interaction (MSI), which could be related to the MS charge transfer. MSI-induced changes in the Pd–CO interaction are much more important in the case of Pd films deposited on metallic supports. The combined X-ray photoelectron (XPS), ultra-violet photoelectron spectroscopy (UPS) and CO-TPD studies have been performed on Pd overlayers on transition metals Ta [15], Mo [16], W, Re, Ru [17], Rh, Pt [17,18], noble metals, such as Au [19], and Al [20–29]. The common feature of Pd deposited on Al and transition metals was the shift of Pd core levels and the center of Pd d-band toward higher binding energies, due to the formation of a bimetallic bond. Originally, the deep-lying Pd 4d-states of Pd–Al were explained in terms of weak Pd–Al interaction giving atomic-like 4d 10 5s 0 electronic configuration [20,21,23,24]. In a second, more recent model the valence and core level BE shift has been attributed, in contrary, to a strong MS interaction [22,26] as a consequence of d–s,p rehybridization and d-electron charge transfer toward the Al substrate [27–29]. For the metal–CO interaction it was shown that the d-band centroid shift, away from the 2p* CO orbitals, leaded to a weakening of the Pd 4d–CO 2p* bonding and to the decrease of CO desorption temperature [30]. In order to explain a possible bimetallic Pd–Al effect, we compare the properties of Pd /Al 2 O 3 , Pd / Al and bulk Pd. XPS and ISS results show a strong interaction between Pd and Al leading to the formation of PdAl alloy of noble metal-like electronic structure. The CO adsorption–desorption studies on Pd /Al systems show an important decrease in the CO desorption temperature accompanied by partial CO dissociation.
2. Experimental XPS and ISS experiments were performed using the Omicron EA 125 multichannel hemispherical analyzer and a dual Al / Mg X-ray source. The substrate cleanliness, stoichiometry and amount of
deposited Pd were checked by XPS. The deposited layer morphology on alumina was investigated using QUASESE software (the software package for quantitative XPS of surface nanostructures by analysis of the peak shape and background). The principles of the method were explained in [31,32]. TPD experiment was carried out using the quadrupole mass spectrometer with a programmable sample heating in the 100–800 K range. The (0001) a-alumina substrate was prepared by heating in the air (at 1620 K for 2 h) and by argon ion bombardment (500 eV, 1 mA cm 22 , 5 min). The Al- and Pd-poly-crystalline samples were cleaned by cycles of argon ion bombardment (900 eV, 2 mA cm 22 ) and heating in vacuum at 900 K. The Pd thin film, equivalent to 1 ML, were deposited on the samples in situ at room temperature using the micro electron beam evaporation source (MEBES) [33] which enables the evaporation rate to be controlled by monitoring the Pd 1 ion current.
3. Results
3.1. Pd–substrate interaction study In order to elucidate the difference between the Pd /Al and Pd /Al 2 O 3 systems, we deposited the equivalent of 1 ML of Pd, at room temperature, on the air annealed (0001)a-Al 2 O 3 and on the argon ion sputtered Al substrates. Using the Pd 3d 5 / 2 peak and background form analysis by the QUASES software, it has been found that Pd formed three-dimensional islands on the alumina substrate. The Pd growth mode on the Al substrate was different, which can be seen in Fig. 1, where the results of the ion scattering spectroscopy (ISS), performed using the 2 keV He 1 ion beam, are shown. The Pd /Al intensity ratios, directly indicating the relative Pd /Al coverage, were 1.7 and 0.15, respectively, which confirmed the earlier observations of Pd–Al alloy formation accompanied by the Pd sub-surface diffusion [22,26]. The XPS studies, obtained by using Mg Ka radiation, are shown in Figs. 2 and 3, where the valence and core Pd3d levels, and the MNN Auger peaks are presented. For comparison the same features for bulk Pd can be seen in Fig. 4. The valence
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Fig. 1. Comparison of the ISS spectra for Pd / aluminum and Pd / a-Al 2 O 3 systems.
Fig. 2. XPS spectra of the a-Al 2 O 3 supported Pd. Top panel: Auger transitions M 4( 5 ) N 45 N 45 and 3d 3 / 2 core level lines of Pd. Lower panel: valence band.
band spectrum in Fig. 2 shows the formation of the Pd bulk-like d-band close to the zero BE (see Fig. 4), that overlaps the alumina valence band. Similarly the Pd3d peak energies for alumina supported particles (Fig. 2) are close to the bulk values (Fig. 4). The small difference between the ‘particle’ and ‘bulk’ BE is given by the general trend of BE variations, which is high for the small clusters and decreases to the bulk value as the number of cluster atoms increases. It is given by the size-dependent reduction of screening effects and final-state energy variation, due to particle charging in the case of poorly conductive substrate [34–36]. The effects of Pd /Al interaction can be seen in Fig. 3: the 3.1 eV shift of the Pd 4d-centroid (comparing to the 2.7 eV shift reported in [28,29] and 2.5 eV in [24] for Al (111)), and the 2 eV shift of the
Pd 3d peak (1.8 eV in [28,29]) to higher BE, relative to the bulk values, due to the strong hybridization of the Pd-d and Al-s states. In Figs. 2–4 it can be seen that also the Pd MNN Auger transition depends on Pd–substrate interaction. It can be decomposed into two elemental, 5 eV different, M 5 N 4,5 N 4,5 and M 4 N 4,5 N 4,5 peaks. The Auger kinetic energy, related to the Fermi level, can be roughly calculated using the first approximation formula, KE(MNN)5BE(M)2BE(N)2BE(N). The difference of several eV between the calculated and measured KE is due to the neglect of the hole–hole interaction energy and the two-hole relaxation energy, which should be considered in the exact calculations. However, this simple approach explains well the relationship, which can be deduced from Figs. 3 and 4, for the Pd Auger and Pd 4d-band
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Fig. 3. XPS spectra of the Pd /Al alloy. Top panel: Auger transitions M 4( 5 ) N 45 N 45 and 3d 3 / 2 core level lines of Pd. Lower panel: valence band.
energy shift DKE(MNN)52DBE(N), because both M and N levels move in the similar manner giving D[BE(M)2BE(N)]50. The second effect, characteristic for the Pd–Al alloy formation, is the separation of M 4 - and M 5 -related components of the MNN Auger transition. Figs. 2–4 show that this behavior is directly linked to the width of the involved (M) and (N) peaks. The narrower the alloy peaks are, the more the MNN Auger peak is structured.
3.2. CO adsorption In order to understand the influence of the Pd–Al interaction on CO adsorption, we investigated CO adsorption–desorption process on Pd /Al by means of TPD and XPS. For comparison, we also performed
Fig. 4. XPS spectra of Pd bulk. Top panel: Auger transitions M 4( 5 ) N 45 N 45 and 3d 3 / 2 core level lines of Pd. Lower panel: valence band.
CO desorption studies, for the same exposures, on a bare Al substrate. TPD results showed that CO desorption temperature decreased substantially for the Pd–Al alloy. Since the CO TPD features for the bulk Pd and Pd / a-Al 2 O 3 exhibited maximum at 475 K [1,5,8], the Pd–Al alloy possessed the maximum at 350 K. This variation of CO adsorption properties can be explained by the formation of a noble metal-like alloy characterized by a lower-lying d-band and a low density of states at the Fermi level, which weakens the Pd 4d–CO 2p* bonding interactions. While it was well established that CO adsorbs molecularly on the bulk and a-Al 2 O 3 supported Pd
´ et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 327 – 332 V. Matolın
[1,5], the XPS studies of the CO adsorption on the Pd /Al alloy showed a different behavior. The interaction of CO with Pd was determined from 1s line of carbon in photoelectron spectra that exhibited three CO-related components. The production of an atomic-like carbon was monitored as a rise of the C 1s signal (with peak at 285 eV) while the molecularly adsorbed CO exhibited the intensity at BE of 287 eV. The spectra exhibited also a peak appearing at 282 eV, which was associated with a metal–carbide species. Fig. 5 illustrates the result of the decomposition of the C 1s-related peaks as a function of the CO adsorption / desorption cycles. Two exposures of 10 Langmuirs (10310 26 Torr s) at room temperature were followed by the desorption heating up to 550 K. The periodic ‘CO peak’ intensity variations, related to the intensity of Pd3d 5 / 2 peak, corresponded to concentration variations of the adsorbed CO molecules during the adsorption–desorption cycles. The most important result of this study was the continuous increase in the atomic-carbon intensity, indicating a partial CO dissociation within both adsorption and desorption processes. Since the ‘reference’ experiment on a clean Al substrate, performed before the Pd deposition at the same conditions, did not show any carbon intensity increase, we can conclude that Pd, interacting with aluminum, exhibited a partial CO dissociation, which accompanied the decrease in the CO-desorption activation energy. Fig. 5 also shows, that the deposited carbon
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is partially transformed, during the cycles, to the metal–carbide form. One can conclude that the Pd d-Al s,p charge transfer, which is accompanied by the Pd–Al work function decrease [28,29], can enhance the 2p* CO antibonding orbital filling by delocalized s,p electrons leading to the weakening of the C–O bond.
4. Summary The Pd / alumina and Pd /Al 2 O 3 interaction can be well characterized by means of XPS. Pd on the a-Al 2 O 3 substrate exhibits the Pd-bulk-like XPS features indicating a weak MS interaction. This is in agreement with the well-established statement that a-Al 2 O 3 is a relatively inert catalyst support. On the other hand Pd is strongly interacting with metallic Al, forming Pd–Al surface alloy, for which the Pd–Al intermixing and a strong electronic interaction are characteristic. The alloy formation is accompanied by the increase of the core- and valence-level BE. The significant narrowing of the valence d-band leads to the narrowing and structuring of the Pd MNN Auger peak, which moves towards lower KE. The Pd–Al alloying decreases simultaneously the CO desorption and dissociation energy.
Acknowledgements This work was supported by the Czech Grant Agency under the Grants No. 202 / 99 / 1714, and by the Czech Ministry of Education, Youth and Sports under the Project No. VS97116.
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Fig. 5. Variations of XPS relative intensities of C 1s peak components with CO adsorption–desorption cycles.
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