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Surface and subsurface oxygen adsorbed on Pt(111)
Surface Science 93 (1980) L147-L150 0 North-Holland Publishing Company
SURFACE SCIENCE LETTERS SURFACE AND SUBSURFACE OXYGEN ADSORBED ON Pt( 111)
Horst NIEHUS and George COMSA Institut fiir Grenzfltichenforschung D-51 7 Jiilich, Germany
und Vakuumphysik,
Kernforschungsanlage
Jiilich,
Received 6 November 1979
Low energy ion scattering (IS) was used to locate the oxygen adsorbed on Pt(ll1) in the “chemisorbed” and in the “oxide” state. The IS spectra show that while the “chemisorbed” oxygen is located in the topmost surface layer, this layer consists only of Pt atoms in the “oxide” case.
There is strong experimental evidence for the existence of two states of oxygen atoms on platinum surfaces [l-8]. The main parameter which determines whether one or the other state is obtained is the surface temperature during the O2 exposure. With temperatures below 800 K the “chemisorbed” state, with temperatures above 800 K the state usually named “oxide” state is obtained [3,4]. The two states can be easily distinguished by their dissimilar properties, as summarized in table 1. The features presented in table 1 correspond mainly to an amount of oxygen in the “oxide” state comparable to the saturation coverage in the chemisorbed state. The significance of the distinctive properties of oxidized Pt for catalysis was already emphasized [4,5]. McCabe and Schmidt [5] have proposed three models for the “oxide” state which could explain its particular properties. Recently, Smith, Biberian and Somorjai [4] have summarized the models as follows: “( 1) the formation of a surface layer of oxide results in a change in the electronic structure of the surface platinum atoms; (2) strongly adsorbed oxygen atoms are active in compound formation with other adsorbates; and (3) oxidation of the platinum surface results in a reconstruction of the surface atoms”. Smith et al. [4] believe that their catalytic measurements favour the first model. They explain the change in the electronic properties of the surface Pt atoms by resuming a former hypothesis by Kikuchi et al. [l] : the oxidation leads to adsorbed oxygen atoms beneath the surface platinum atoms. Smith et al. find an indication for this behaviour in the work function changes as shown in table 1. We address here just the question, whether after low coverage oxidation of a Pt surface the oxygen atoms are really below the Pt surface atoms or not. We analysed the Pt(lll) surface by low energy He ion scattering (IS). Due to beam attenuation and neutralization effects this method is sensitive only to the atoms in the first L147
H. Niehus, G. Comsa / Surface and subsurface oxygen on Pt(1 I I)
L148
Table 1 “Chemisorbed” Generation Desorption Reaction
Binding
Catalytic
by 02 exposure
at
with Hz and CO
energies
of H, and CO
activity
Main IR bands of adsorbed
CO
Position of oxygen Auger peaks (chemical shift) Work function change with respect to clean Pt
oxygen
7-c 800 K [3,4] Slowly at T > 550 K, completelyatT= 1OOOK [3] Removed even at room temperature [6,3], “reactive” Like on clean Pt (because oxygen is removed in presence of Hz and CO) Like on clean Pt (because oxygen is removed in presence of Hz and CO) 2060-80 cm-’ like on clean Pt (see above) [ 11, “Type I” 496and517eV[3] (A = 6 eV) +0.5 eV [6,7]
“Oxide” T>800K [3,4] ‘slow decomposition at T 1200 K [3] Not removed even at 1000 K [31, “non-reactive” New states with higher binding energies than on clean Pt [S] Larger activity and changed selectivity compared to clean Pt [41 New bandat “Type II”
2120 cm-l
[I],
491 and 511 eV [3] (A z 6 eV) -1 eV [8]
surface layer 191. Accordingly, a univocal answer could be expected. Experiments were performed in a vacuum vessel (base pressure 5 X 10-l ’ mbar) with Auger spectroscopy (AES), LEED and ion sputtering facilities [lo]. For the IS measurements an ion beam source (Atomica) and a spherical electrostatic energy analyser (Leybold-Heraeus) with an angular resolution of +-1S” were added. Intensity and energy of the incident He4 beams were 40 nA and 1960 eV respectively. The scattered ions were analysed in the specular direction. Measurements were performed at two scattering angles 9 = 60” and 90’ and two azimuths cp= 0” and 30’ (incident plane in the [011] and [112] directions of the Pt(ll1) surface, respectively). During measurements the sample was at room temperature. The Pt(ll1) crystal was cleaned by alternate heating in O2 up to 1200 K, flashing in vacuum and sputtering. The AES spectrum of the “clean” surface can be seen in fig. la. Apart from a faint carbon peak, the other features belong to Pt. The chemisorbed oxygen was obtained by exposing the cold Pt surface (room temperature or below) to O2 at p = 10e6 mbar. Peak-to-peak 05r0/PtZa7 AES ratios up to 0.5 were obtained. The measurements were performed mainly with 0510/Pt237 ratios around 0.4, obtained after a 2000 L 02-exposure (fig. lb). At this coverage the LEED (2 X 2) structure was already clearly visible. On the other hand, the “oxide” was produced by exposing the hot Pt surface (1100 K) to 0, at p = 1 X 10e6 mbar. After 15 min exposure an 05e3 /Ptzx7 ratio of the same value, 0.4,
H. Niehus, G. Coma /Surface
and subsurface oxygen on Pt(l I I)
L149
Ep= 2 keV Un=5 v,
a) clean Pt
b) chemisorbed oxygen
Cl oxide
Pt237 I
100
200
300
LOO 500 EleV
Fig. 1. AES spectra.
was obtained (fig. 1c). The obvious shift of the oxygen Auger peak (figs. 1b and Ic) shows that the two expected states of the oxygen atoms are indeed obtained. We verified also another property of the two states listed in table 1: while the Auger spectrum of the oxide (fig. lc) remained unchanged even after heating at 1300 K, the chemisorbed oxygen (fig. lb) disappeared after heating above 400 K and a “clean” spectrum (fig. la) was obtained. The removal of the chemisorbed oxygen was probably due to the reaction with residual Hz and CO. Let us now compare the three IS spectra in figs. 2a, 2b and 2c obtained from the Pt surfaces characterized by the AES spectra in figs. la, 1b and lc respectively. The main feature of fig. 2a, obtained from the “clean” surface, is the Pt peak at the energy calculated for a single collision, E = 1920 eV (see, e.g., ref. [9]). The chemisorption of at most one 0 atom for two Pt surface atoms (2 X 2 structure) has a dramatic effect (fig. 2b): the Pt peak is reduced by more than a factor of eight and the 0 peak appears at the expected energy E = 1520 eV. The principal piece of evidence for the question we are addressing here is fig. 2c: the IS spectrum of the “oxide” surface is almost identical to the spectrum of the “clean” surface (fig. 2a). The heights of the Pt peaks are equal in the limit of the experimental errors and the background in the case of the “oxide” is even smaller. The spectra obtained at the other azimuth and scattering angles led to similar results. We think that this is a convincing proof that the first layer of the “oxide” surface consists only of Pt atoms. The assumption that “oxidation leads to adsorbed oxygen atoms beneath
L150
H. Niehus, G. Comsa /Surface
a) clean Pt
1
1.~
1.6
1.8
and subsurface oxygen on Pt(l II)
b) chemisarbed oxygen
20
1
1L
16
1.8
cl oxide
201
1.L
16
18 E
20
I keV
Fig. 2. IS spectra of the surfaces characterized in fig. 1.
the surface platinum atoms” [4] suggested by the decrease in work function is thus confirmed. Final remark: the dramatic difference between the IS spectra (figs. 2b and 2c) obtained from surfaces with the same O/Pt Auger ratio is a striking demonstration for the surface sensitivity of IS when compared to AES. Therefore, IS easily distinguishes between surface and subsurface adsorbates.
References [l] [2] [3] [4] [S] [6] [7] [S]
E. Kikuchi, P.C. Flynn and S.E. Wanke, J. Catalysis 34 (1974) 132. R. Ducros and R.P. Merrill, Surface Sci. 55 (1976) 227. T. Matsushima, D.B. Almy and J.M. White, Surface Sci. 67 (1977) 89. C.E. Smith, J.P. Biberian and G.A. Somorjai, J. Catalysis 57 (1979) 426. R.W. McCabe and L.D. Schmidt, Surface Sci. 60 (1976) 85; 65 (1977) 189. C.R. Helms, H.P. Bonzel and S. Kelemen, J. Chem. Phys. 65 (1976) 1773. D.M. Collins, J.B. Lee and W.E. Spicer, Surface Sci. 55 (1976) 389. W.H. Weinberg, D.R. Monroe, V. Lampton and R.P. Merrill, J. Vacuum Sci. Technol. 14 (1977) 444. (91 W. Heiland and E. Taglauer, Surface Sci. 68 (1977) 96. [lo] H. Niehus, Surface Sci. 80 (1979) 245.
SURFACE SCIENCE LETTERS SURFACE AND SUBSURFACE OXYGEN ADSORBED ON Pt( 111)
Horst NIEHUS and George COMSA Institut fiir Grenzfltichenforschung D-51 7 Jiilich, Germany
und Vakuumphysik,
Kernforschungsanlage
Jiilich,
Received 6 November 1979
Low energy ion scattering (IS) was used to locate the oxygen adsorbed on Pt(ll1) in the “chemisorbed” and in the “oxide” state. The IS spectra show that while the “chemisorbed” oxygen is located in the topmost surface layer, this layer consists only of Pt atoms in the “oxide” case.
There is strong experimental evidence for the existence of two states of oxygen atoms on platinum surfaces [l-8]. The main parameter which determines whether one or the other state is obtained is the surface temperature during the O2 exposure. With temperatures below 800 K the “chemisorbed” state, with temperatures above 800 K the state usually named “oxide” state is obtained [3,4]. The two states can be easily distinguished by their dissimilar properties, as summarized in table 1. The features presented in table 1 correspond mainly to an amount of oxygen in the “oxide” state comparable to the saturation coverage in the chemisorbed state. The significance of the distinctive properties of oxidized Pt for catalysis was already emphasized [4,5]. McCabe and Schmidt [5] have proposed three models for the “oxide” state which could explain its particular properties. Recently, Smith, Biberian and Somorjai [4] have summarized the models as follows: “( 1) the formation of a surface layer of oxide results in a change in the electronic structure of the surface platinum atoms; (2) strongly adsorbed oxygen atoms are active in compound formation with other adsorbates; and (3) oxidation of the platinum surface results in a reconstruction of the surface atoms”. Smith et al. [4] believe that their catalytic measurements favour the first model. They explain the change in the electronic properties of the surface Pt atoms by resuming a former hypothesis by Kikuchi et al. [l] : the oxidation leads to adsorbed oxygen atoms beneath the surface platinum atoms. Smith et al. find an indication for this behaviour in the work function changes as shown in table 1. We address here just the question, whether after low coverage oxidation of a Pt surface the oxygen atoms are really below the Pt surface atoms or not. We analysed the Pt(lll) surface by low energy He ion scattering (IS). Due to beam attenuation and neutralization effects this method is sensitive only to the atoms in the first L147
H. Niehus, G. Comsa / Surface and subsurface oxygen on Pt(1 I I)
L148
Table 1 “Chemisorbed” Generation Desorption Reaction
Binding
Catalytic
by 02 exposure
at
with Hz and CO
energies
of H, and CO
activity
Main IR bands of adsorbed
CO
Position of oxygen Auger peaks (chemical shift) Work function change with respect to clean Pt
oxygen
7-c 800 K [3,4] Slowly at T > 550 K, completelyatT= 1OOOK [3] Removed even at room temperature [6,3], “reactive” Like on clean Pt (because oxygen is removed in presence of Hz and CO) Like on clean Pt (because oxygen is removed in presence of Hz and CO) 2060-80 cm-’ like on clean Pt (see above) [ 11, “Type I” 496and517eV[3] (A = 6 eV) +0.5 eV [6,7]
“Oxide” T>800K [3,4] ‘slow decomposition at T 1200 K [3] Not removed even at 1000 K [31, “non-reactive” New states with higher binding energies than on clean Pt [S] Larger activity and changed selectivity compared to clean Pt [41 New bandat “Type II”
2120 cm-l
[I],
491 and 511 eV [3] (A z 6 eV) -1 eV [8]
surface layer 191. Accordingly, a univocal answer could be expected. Experiments were performed in a vacuum vessel (base pressure 5 X 10-l ’ mbar) with Auger spectroscopy (AES), LEED and ion sputtering facilities [lo]. For the IS measurements an ion beam source (Atomica) and a spherical electrostatic energy analyser (Leybold-Heraeus) with an angular resolution of +-1S” were added. Intensity and energy of the incident He4 beams were 40 nA and 1960 eV respectively. The scattered ions were analysed in the specular direction. Measurements were performed at two scattering angles 9 = 60” and 90’ and two azimuths cp= 0” and 30’ (incident plane in the [011] and [112] directions of the Pt(ll1) surface, respectively). During measurements the sample was at room temperature. The Pt(ll1) crystal was cleaned by alternate heating in O2 up to 1200 K, flashing in vacuum and sputtering. The AES spectrum of the “clean” surface can be seen in fig. la. Apart from a faint carbon peak, the other features belong to Pt. The chemisorbed oxygen was obtained by exposing the cold Pt surface (room temperature or below) to O2 at p = 10e6 mbar. Peak-to-peak 05r0/PtZa7 AES ratios up to 0.5 were obtained. The measurements were performed mainly with 0510/Pt237 ratios around 0.4, obtained after a 2000 L 02-exposure (fig. lb). At this coverage the LEED (2 X 2) structure was already clearly visible. On the other hand, the “oxide” was produced by exposing the hot Pt surface (1100 K) to 0, at p = 1 X 10e6 mbar. After 15 min exposure an 05e3 /Ptzx7 ratio of the same value, 0.4,
H. Niehus, G. Coma /Surface
and subsurface oxygen on Pt(l I I)
L149
Ep= 2 keV Un=5 v,
a) clean Pt
b) chemisorbed oxygen
Cl oxide
Pt237 I
100
200
300
LOO 500 EleV
Fig. 1. AES spectra.
was obtained (fig. 1c). The obvious shift of the oxygen Auger peak (figs. 1b and Ic) shows that the two expected states of the oxygen atoms are indeed obtained. We verified also another property of the two states listed in table 1: while the Auger spectrum of the oxide (fig. lc) remained unchanged even after heating at 1300 K, the chemisorbed oxygen (fig. lb) disappeared after heating above 400 K and a “clean” spectrum (fig. la) was obtained. The removal of the chemisorbed oxygen was probably due to the reaction with residual Hz and CO. Let us now compare the three IS spectra in figs. 2a, 2b and 2c obtained from the Pt surfaces characterized by the AES spectra in figs. la, 1b and lc respectively. The main feature of fig. 2a, obtained from the “clean” surface, is the Pt peak at the energy calculated for a single collision, E = 1920 eV (see, e.g., ref. [9]). The chemisorption of at most one 0 atom for two Pt surface atoms (2 X 2 structure) has a dramatic effect (fig. 2b): the Pt peak is reduced by more than a factor of eight and the 0 peak appears at the expected energy E = 1520 eV. The principal piece of evidence for the question we are addressing here is fig. 2c: the IS spectrum of the “oxide” surface is almost identical to the spectrum of the “clean” surface (fig. 2a). The heights of the Pt peaks are equal in the limit of the experimental errors and the background in the case of the “oxide” is even smaller. The spectra obtained at the other azimuth and scattering angles led to similar results. We think that this is a convincing proof that the first layer of the “oxide” surface consists only of Pt atoms. The assumption that “oxidation leads to adsorbed oxygen atoms beneath
L150
H. Niehus, G. Comsa /Surface
a) clean Pt
1
1.~
1.6
1.8
and subsurface oxygen on Pt(l II)
b) chemisarbed oxygen
20
1
1L
16
1.8
cl oxide
201
1.L
16
18 E
20
I keV
Fig. 2. IS spectra of the surfaces characterized in fig. 1.
the surface platinum atoms” [4] suggested by the decrease in work function is thus confirmed. Final remark: the dramatic difference between the IS spectra (figs. 2b and 2c) obtained from surfaces with the same O/Pt Auger ratio is a striking demonstration for the surface sensitivity of IS when compared to AES. Therefore, IS easily distinguishes between surface and subsurface adsorbates.
References [l] [2] [3] [4] [S] [6] [7] [S]
E. Kikuchi, P.C. Flynn and S.E. Wanke, J. Catalysis 34 (1974) 132. R. Ducros and R.P. Merrill, Surface Sci. 55 (1976) 227. T. Matsushima, D.B. Almy and J.M. White, Surface Sci. 67 (1977) 89. C.E. Smith, J.P. Biberian and G.A. Somorjai, J. Catalysis 57 (1979) 426. R.W. McCabe and L.D. Schmidt, Surface Sci. 60 (1976) 85; 65 (1977) 189. C.R. Helms, H.P. Bonzel and S. Kelemen, J. Chem. Phys. 65 (1976) 1773. D.M. Collins, J.B. Lee and W.E. Spicer, Surface Sci. 55 (1976) 389. W.H. Weinberg, D.R. Monroe, V. Lampton and R.P. Merrill, J. Vacuum Sci. Technol. 14 (1977) 444. (91 W. Heiland and E. Taglauer, Surface Sci. 68 (1977) 96. [lo] H. Niehus, Surface Sci. 80 (1979) 245.