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CO adsorption on Pd(110)
Surface Science 402–404 (1998) 595–598
CO adsorption on Pd(110) I.Z. Jones, R.A. Bennett, M. Bowker * Department of Chemistry, Catalysis Research Centre, University of Reading, Whiteknights, Reading RG6 6AD, UK Received 1 August 1997; accepted for publication 21 October 1997
Abstract CO sticking, adsorption and desorption on Pd(110) has been studied using a molecular beam reactor and real time temperatureprogrammed X-ray photoelectron spectroscopy (TPXPS ). The initial sticking and total uptake decrease with increasing temperature. The C 1s and O 1s binding energies were followed during adsorption, and shifted to higher energy with increasing exposure. TPXPS reveals a decrease in C 1s and O 1s signal intensity at ~340 K which coincides with the a CO desorption, associated with substrate 3 reconstruction. Binding energy shifts in the C 1s region do not appear to be correlated with this reconstruction, and are thus only dependent upon coverage. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Carbon monoxide; Low index single crystal surfaces; Palladium; Sticking; Surface relaxation and reconstruction; Synchrotron radiation photoelectron spectroscopy; Thermal desorption; X-ray photoelectron spectroscopy
1. Introduction The understanding of CO adsorption and desorption is essential to automobile exhaust gas catalysis, where CO is removed by reaction with O to form CO . CO adsorption on Pd(110) single 2 2 crystals has been studied extensively using many techniques such as TPD [1], RAIRS [2–4], LEED [5,6 ] and PD [7]. TPD results show five CO desorption peaks, of which the sharp a peak at 3 338 K is associated with a (2×1) to (4×2) transition which involves a missing-row reconstruction [1]. (1×1), (4×2) and (2×1)p2mg ordered phases are observed with increasing CO adsorption at 300 K by LEED [4]. The missing-row reconstruction is not observed when CO is adsorbed at 180 K, however, upon heating to above 250 K, the * Corresponding author. Fax: (+44) 1189 316632; e-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 00 0 5 -3
intermediate coverage phases irreversibly convert to (4×2). Thus the formation of (4×2) and the missing row reconstruction is kinetically hindered at low temperature (180 K ). In this work a molecular beam reactor and XPS have been used to probe the CO/Pd system.
2. Experimental The molecular beam reactor [8] consists of a molecular beam attached to the main analysis chamber, which is equipped with facilities for argon ion sputtering, LEED and AES, and a quadrupole mass spectrometer. The molecular beam produces a beam of molecules which impinge on the surface with a diameter of 2.9 mm. It has an in beam pressure of around 1.6×10−7 mbar at the surface and a flux of approximately 5×1013 molecules cm−2 s−1 when 20 mbar of gas
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I.Z. Jones et al. / Surface Science 402–404 (1998) 595–598
is introduced to the effusion chamber. The XPS experiments were performed at the SuperESCA beamline at ELETTRA [9]. The fast acquisition time enables adsorption and desorption to be followed in real time. The XPS system is equipped with a VSW class 150 16-channel electron energy analyser, LEED and a mass spectrometer. Both UHV chambers had a base pressure of <2×10−10 mbar and the same sample cleaning techniques. Cycles of argon ion bombardment, annealing and heating in oxygen at 830 K [10] were used in both chambers. Surface cleanliness was verified using CO desorption and Auger surface analysis in the former system and XPS in the latter system. Pd 3d at 335.1 eV was used to 5/2 calibrate the binding energy scale. Spectra were taken every ~10 s and were fitted with Doniach–Sunjicˇ [11] line shapes, the resulting intensity and position being plotted as functions of temperature and exposure. For O 1s the overlap with Pd 3p was removed by scaling and subtract3/2 ing a clean Pd 3p peak from the data. 3/2 3. Results and discussion 3.1. Sticking probability of CO The sticking probability of CO at various Pd temperatures with respect to coverage is shown in Fig. 1. The initial sticking probability of CO is greater at lower temperatures, i.e. 0.5 at 315 K compared to 0.3 at 472 K. The desorption temperature of CO is 470 K, and thus at temperatures
Fig. 1. The sticking probability of CO on clean Pd(110) with respect to coverage at four sample temperatures.
nearing this value there is less net sticking, which is a result of desorption competing with adsorption. The sticking probability falls with increasing coverage. This is because as the surface becomes increasingly covered with CO there are less vacant sites on which the CO can adsorb, and therefore the probability of CO finding a vacant site decreases. The decrease in sticking is non-linear, which is a consequence of precursor state effects [12]. The C 1s peak areas are shown as a function of exposure in Fig. 2a for a CO uptake dosed with 2×10−9 mbar CO at a sample temperature of 310 K. Each point corresponds to one spectrum. The C 1s signal increases linearly with exposure, the apparent discrepancies with the molecular beam sticking measurements are possibly an effect of forward focusing in the XPS. 3.2. CO adsorption and desorption: C 1s XPS Fig. 2b shows the C 1s spectra during CO uptake on clean Pd(110), spectra being taken every ~10 s. The sample was continually exposed to 2×10−9 mbar of CO at 137.5 K. The results show a gradual shift in binding energy of the C 1s signal with increasing exposure. Initially, at low coverage the C 1s signal has a binding-energy value of 285.64 eV. As the coverage is increased there is a shift to higher binding energy, and at saturation the binding energy has shifted by 0.26 eV. The observed shift in binding energy with increasing exposure can be rationalised in terms of bonding. At low coverage the C–Pd bond is strong, a consequence of strong backbonding (where Pd d electrons are transferred into the p* orbital of C ) and transfer of electrons from the 5s orbital of C to the Pd d orbitals [13]. As the coverage is increased, the strength of the C–Pd bond is weakened [14]. This is due to a decrease in backdonation of charge from Pd to the p* orbital of C, resulting in a decrease of localisation of charge on the atoms. This leads to a decrease in the screening effect by these localised valence electrons, which in turn results in a higher binding energy. Fig. 2c is a TPXP spectrum of CO from the saturated (2×1)p2mg surface, showing the change in binding energy and coverage with increasing temperature. The surface was dosed at 137.5 K
I.Z. Jones et al. / Surface Science 402–404 (1998) 595–598
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(4×2) overlayer, which is (1×2) missing row reconstructed. 3.3. CO adsorption and desorption: O 1s XPS Fig. 3a shows the O 1s binding energy and coverage during CO uptake on clean Pd(110), spectra being taken every ~10 s. The sample was continually exposed to 2×10−9 mbar of CO at 133 K. The results show a continual shift in binding energy of the O 1s signal with increasing exposure. Initially, at low coverage the O 1s signal has a binding energy value of 532.2 eV. As the coverage is increased there is a shift to higher binding energy, and at saturation the binding energy has shifted by 0.34 eV, a shift similar to that observed for C 1s. Fig. 3b is an O 1s TPXP spectrum from a CO-saturated surface. CO was dosed at 133 K and the ramp rate was 0.3 K s−1. Fig. 3b shows the decrease in coverage with increasing temper-
Fig. 2. (a) The effect of increasing exposure on the peak area of the C 1s signal for a CO uptake dosed with 2×10−9 mbar CO at 310 K. (b) C 1s spectra of a CO uptake at 137.5 K, dosed with 2×10−9 mbar CO. (c) The coverage (filled circles) and binding energy (empty circles) changes for C 1s TPXPS from a CO-saturated surface at 137.5 K. Ramp rate=0.5 K s−1.
and the temperature was increased by 0.5 K s−1. The binding energy shift is 0.26 eV to lower energy, an identical difference in binding energy to that observed in Fig. 2b, and can again be explained in terms of bonding. The binding energy shifts from 285.89 to 285.65 eV at 300 K. This shift mainly occurs after the coverage has fallen to 0.7 ML, which equates to the onset of the formation of the
Fig. 3. (a) The coverage (filled circles) and binding energy (empty circles) of O 1s against CO exposure for an uptake at 133 K, dosed with 2×10−9 mbar CO. (b) O 1s TPXPS from a CO-saturated surface dosed at 133 K at a heating rate of 0.3 K s−1, showing the effect of temperature on the CO coverage.
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I.Z. Jones et al. / Surface Science 402–404 (1998) 595–598
ature. A discussion of the detailed binding energy changes for these data is outside the scope of this paper, and will be discussed elsewhere [15]. At 340 K there is a dip in the rate of coverage decrease, which is a consequence of desorption of the a CO state. This a state has been shown to 3 3 be associated with the onset of the missing row reconstruction [1].
4. Conclusion The sticking probability of CO on Pd(110) has been measured for a range of temperatures, and shows a trend towards a lower initial sticking coefficient and total uptake with increasing crystal temperature. The adsorption and desorption of CO were also followed by fast XPS at the C 1s and O 1s peaks. The uptake curves reveal a continuous shift in the C 1s and O 1s binding energies to higher energy with exposure. TPXPS at both C 1s and O 1s shows a decrease in signal intensity at ~340 K which coincides with the a CO desorp3 tion, which in turn is associated with substrate reconstruction. Binding energy shifts in the C 1s region do not appear to be correlated with this reconstruction, and are thus only dependent upon coverage.
Acknowledgements The authors would like to acknowledge F. Esch, A. Baraldi, S. Lizzit and G. Comelli, and to thank
the EU, EPSRC and Johnson Matthey plc for their financial support.
References [1] J.-W. He, P.R. Norton, J. Chem. Phys. 89 (1988) 1170. [2] R. Raval, M.A. Harrison, D.A. King, Surf. Sci. 211/212 (1989) 61. [3] R. Raval, S. Haq, G. Blyholder, D.A. King, J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 629. [4] R. Raval, S. Haq, M.A. Harrison, G. Blyholder, D.A. King, Chem. Phys. Lett. 167 (1990) 391. [5] P. Hu, L. Morales de la Garza, R. Raval, D.A. King, Surf. Sci. 249 (1991) 1. [6 ] A. Wander, P. Hu, D.A. King, Chem. Phys. Lett. 201 (1993) 393. [7] A. Locatelli, B. Brena, G. Comelli, S. Lizzit, G. Paolucci, R. Rosei, Phys. Rev. B 54 (4) (1996) 2839. [8] M. Bowker, P.D.A. Pudney, C.J. Barnes, J. Vac. Sci. Technol. A 8 (2) (1990) 816. [9] A. Baraldi, M. Barnaba, B. Brena, D. Cocco, G. Comelli, S. Lizzit, G. Paolucci, R. Rosei, J. Electron. Spectrosc. Relat. Phenom. 76 (1995) 145. [10] M. Milun, P. Pervan, M. Vajic, K. Wandelt, Surf. Sci. 211 (1989) 887. [11] J.J. Joyce, M. Del Giudice, J.H. Weaver, J. Electron. Spectrosc. Relat. Phenom. 49 (1989) 31. [12] M. Bowker, Langmuir 7 (1991) 2534. [13] B. Hammer, O.H. Nielsen, J.K. Nørskov, Catal. Lett. 46 (1997) 31. [14] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994. [15] I.Z. Jones, R.A. Bennett, M. Bowker, F. Esch, A. Baraldi, S. Lizzit, G. Comelli, to be published.
CO adsorption on Pd(110) I.Z. Jones, R.A. Bennett, M. Bowker * Department of Chemistry, Catalysis Research Centre, University of Reading, Whiteknights, Reading RG6 6AD, UK Received 1 August 1997; accepted for publication 21 October 1997
Abstract CO sticking, adsorption and desorption on Pd(110) has been studied using a molecular beam reactor and real time temperatureprogrammed X-ray photoelectron spectroscopy (TPXPS ). The initial sticking and total uptake decrease with increasing temperature. The C 1s and O 1s binding energies were followed during adsorption, and shifted to higher energy with increasing exposure. TPXPS reveals a decrease in C 1s and O 1s signal intensity at ~340 K which coincides with the a CO desorption, associated with substrate 3 reconstruction. Binding energy shifts in the C 1s region do not appear to be correlated with this reconstruction, and are thus only dependent upon coverage. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Carbon monoxide; Low index single crystal surfaces; Palladium; Sticking; Surface relaxation and reconstruction; Synchrotron radiation photoelectron spectroscopy; Thermal desorption; X-ray photoelectron spectroscopy
1. Introduction The understanding of CO adsorption and desorption is essential to automobile exhaust gas catalysis, where CO is removed by reaction with O to form CO . CO adsorption on Pd(110) single 2 2 crystals has been studied extensively using many techniques such as TPD [1], RAIRS [2–4], LEED [5,6 ] and PD [7]. TPD results show five CO desorption peaks, of which the sharp a peak at 3 338 K is associated with a (2×1) to (4×2) transition which involves a missing-row reconstruction [1]. (1×1), (4×2) and (2×1)p2mg ordered phases are observed with increasing CO adsorption at 300 K by LEED [4]. The missing-row reconstruction is not observed when CO is adsorbed at 180 K, however, upon heating to above 250 K, the * Corresponding author. Fax: (+44) 1189 316632; e-mail: [email protected] 0039-6028/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 98 ) 0 00 0 5 -3
intermediate coverage phases irreversibly convert to (4×2). Thus the formation of (4×2) and the missing row reconstruction is kinetically hindered at low temperature (180 K ). In this work a molecular beam reactor and XPS have been used to probe the CO/Pd system.
2. Experimental The molecular beam reactor [8] consists of a molecular beam attached to the main analysis chamber, which is equipped with facilities for argon ion sputtering, LEED and AES, and a quadrupole mass spectrometer. The molecular beam produces a beam of molecules which impinge on the surface with a diameter of 2.9 mm. It has an in beam pressure of around 1.6×10−7 mbar at the surface and a flux of approximately 5×1013 molecules cm−2 s−1 when 20 mbar of gas
596
I.Z. Jones et al. / Surface Science 402–404 (1998) 595–598
is introduced to the effusion chamber. The XPS experiments were performed at the SuperESCA beamline at ELETTRA [9]. The fast acquisition time enables adsorption and desorption to be followed in real time. The XPS system is equipped with a VSW class 150 16-channel electron energy analyser, LEED and a mass spectrometer. Both UHV chambers had a base pressure of <2×10−10 mbar and the same sample cleaning techniques. Cycles of argon ion bombardment, annealing and heating in oxygen at 830 K [10] were used in both chambers. Surface cleanliness was verified using CO desorption and Auger surface analysis in the former system and XPS in the latter system. Pd 3d at 335.1 eV was used to 5/2 calibrate the binding energy scale. Spectra were taken every ~10 s and were fitted with Doniach–Sunjicˇ [11] line shapes, the resulting intensity and position being plotted as functions of temperature and exposure. For O 1s the overlap with Pd 3p was removed by scaling and subtract3/2 ing a clean Pd 3p peak from the data. 3/2 3. Results and discussion 3.1. Sticking probability of CO The sticking probability of CO at various Pd temperatures with respect to coverage is shown in Fig. 1. The initial sticking probability of CO is greater at lower temperatures, i.e. 0.5 at 315 K compared to 0.3 at 472 K. The desorption temperature of CO is 470 K, and thus at temperatures
Fig. 1. The sticking probability of CO on clean Pd(110) with respect to coverage at four sample temperatures.
nearing this value there is less net sticking, which is a result of desorption competing with adsorption. The sticking probability falls with increasing coverage. This is because as the surface becomes increasingly covered with CO there are less vacant sites on which the CO can adsorb, and therefore the probability of CO finding a vacant site decreases. The decrease in sticking is non-linear, which is a consequence of precursor state effects [12]. The C 1s peak areas are shown as a function of exposure in Fig. 2a for a CO uptake dosed with 2×10−9 mbar CO at a sample temperature of 310 K. Each point corresponds to one spectrum. The C 1s signal increases linearly with exposure, the apparent discrepancies with the molecular beam sticking measurements are possibly an effect of forward focusing in the XPS. 3.2. CO adsorption and desorption: C 1s XPS Fig. 2b shows the C 1s spectra during CO uptake on clean Pd(110), spectra being taken every ~10 s. The sample was continually exposed to 2×10−9 mbar of CO at 137.5 K. The results show a gradual shift in binding energy of the C 1s signal with increasing exposure. Initially, at low coverage the C 1s signal has a binding-energy value of 285.64 eV. As the coverage is increased there is a shift to higher binding energy, and at saturation the binding energy has shifted by 0.26 eV. The observed shift in binding energy with increasing exposure can be rationalised in terms of bonding. At low coverage the C–Pd bond is strong, a consequence of strong backbonding (where Pd d electrons are transferred into the p* orbital of C ) and transfer of electrons from the 5s orbital of C to the Pd d orbitals [13]. As the coverage is increased, the strength of the C–Pd bond is weakened [14]. This is due to a decrease in backdonation of charge from Pd to the p* orbital of C, resulting in a decrease of localisation of charge on the atoms. This leads to a decrease in the screening effect by these localised valence electrons, which in turn results in a higher binding energy. Fig. 2c is a TPXP spectrum of CO from the saturated (2×1)p2mg surface, showing the change in binding energy and coverage with increasing temperature. The surface was dosed at 137.5 K
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(4×2) overlayer, which is (1×2) missing row reconstructed. 3.3. CO adsorption and desorption: O 1s XPS Fig. 3a shows the O 1s binding energy and coverage during CO uptake on clean Pd(110), spectra being taken every ~10 s. The sample was continually exposed to 2×10−9 mbar of CO at 133 K. The results show a continual shift in binding energy of the O 1s signal with increasing exposure. Initially, at low coverage the O 1s signal has a binding energy value of 532.2 eV. As the coverage is increased there is a shift to higher binding energy, and at saturation the binding energy has shifted by 0.34 eV, a shift similar to that observed for C 1s. Fig. 3b is an O 1s TPXP spectrum from a CO-saturated surface. CO was dosed at 133 K and the ramp rate was 0.3 K s−1. Fig. 3b shows the decrease in coverage with increasing temper-
Fig. 2. (a) The effect of increasing exposure on the peak area of the C 1s signal for a CO uptake dosed with 2×10−9 mbar CO at 310 K. (b) C 1s spectra of a CO uptake at 137.5 K, dosed with 2×10−9 mbar CO. (c) The coverage (filled circles) and binding energy (empty circles) changes for C 1s TPXPS from a CO-saturated surface at 137.5 K. Ramp rate=0.5 K s−1.
and the temperature was increased by 0.5 K s−1. The binding energy shift is 0.26 eV to lower energy, an identical difference in binding energy to that observed in Fig. 2b, and can again be explained in terms of bonding. The binding energy shifts from 285.89 to 285.65 eV at 300 K. This shift mainly occurs after the coverage has fallen to 0.7 ML, which equates to the onset of the formation of the
Fig. 3. (a) The coverage (filled circles) and binding energy (empty circles) of O 1s against CO exposure for an uptake at 133 K, dosed with 2×10−9 mbar CO. (b) O 1s TPXPS from a CO-saturated surface dosed at 133 K at a heating rate of 0.3 K s−1, showing the effect of temperature on the CO coverage.
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I.Z. Jones et al. / Surface Science 402–404 (1998) 595–598
ature. A discussion of the detailed binding energy changes for these data is outside the scope of this paper, and will be discussed elsewhere [15]. At 340 K there is a dip in the rate of coverage decrease, which is a consequence of desorption of the a CO state. This a state has been shown to 3 3 be associated with the onset of the missing row reconstruction [1].
4. Conclusion The sticking probability of CO on Pd(110) has been measured for a range of temperatures, and shows a trend towards a lower initial sticking coefficient and total uptake with increasing crystal temperature. The adsorption and desorption of CO were also followed by fast XPS at the C 1s and O 1s peaks. The uptake curves reveal a continuous shift in the C 1s and O 1s binding energies to higher energy with exposure. TPXPS at both C 1s and O 1s shows a decrease in signal intensity at ~340 K which coincides with the a CO desorp3 tion, which in turn is associated with substrate reconstruction. Binding energy shifts in the C 1s region do not appear to be correlated with this reconstruction, and are thus only dependent upon coverage.
Acknowledgements The authors would like to acknowledge F. Esch, A. Baraldi, S. Lizzit and G. Comelli, and to thank
the EU, EPSRC and Johnson Matthey plc for their financial support.
References [1] J.-W. He, P.R. Norton, J. Chem. Phys. 89 (1988) 1170. [2] R. Raval, M.A. Harrison, D.A. King, Surf. Sci. 211/212 (1989) 61. [3] R. Raval, S. Haq, G. Blyholder, D.A. King, J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 629. [4] R. Raval, S. Haq, M.A. Harrison, G. Blyholder, D.A. King, Chem. Phys. Lett. 167 (1990) 391. [5] P. Hu, L. Morales de la Garza, R. Raval, D.A. King, Surf. Sci. 249 (1991) 1. [6 ] A. Wander, P. Hu, D.A. King, Chem. Phys. Lett. 201 (1993) 393. [7] A. Locatelli, B. Brena, G. Comelli, S. Lizzit, G. Paolucci, R. Rosei, Phys. Rev. B 54 (4) (1996) 2839. [8] M. Bowker, P.D.A. Pudney, C.J. Barnes, J. Vac. Sci. Technol. A 8 (2) (1990) 816. [9] A. Baraldi, M. Barnaba, B. Brena, D. Cocco, G. Comelli, S. Lizzit, G. Paolucci, R. Rosei, J. Electron. Spectrosc. Relat. Phenom. 76 (1995) 145. [10] M. Milun, P. Pervan, M. Vajic, K. Wandelt, Surf. Sci. 211 (1989) 887. [11] J.J. Joyce, M. Del Giudice, J.H. Weaver, J. Electron. Spectrosc. Relat. Phenom. 49 (1989) 31. [12] M. Bowker, Langmuir 7 (1991) 2534. [13] B. Hammer, O.H. Nielsen, J.K. Nørskov, Catal. Lett. 46 (1997) 31. [14] G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994. [15] I.Z. Jones, R.A. Bennett, M. Bowker, F. Esch, A. Baraldi, S. Lizzit, G. Comelli, to be published.