- Email: [email protected]
A combined EELS-XPS study of molecularly chemisorbed oxygen on silver surfaces: Evidence for superoxo and peroxo species
Surface Science 186 (1987) L575-L580 North-Holland, A m s t e r d a m
L575
S U R F A C E SCIENCE LETTERS A C O M B I N E D E E L S - X P S S T U D Y OF MOLECULARLY C H E M I S O R B E D O X Y G E N O N SILVER SURFACES: EVIDENCE FOR SUPEROXO AND PEROXO SPECIES * K. P R A B H A K A R A N and C.N.R. RAO * * Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India Received 6 March 1987; accepted for publication 22 April 1987
Both superoxo and peroxo species are formed when oxygen is adsorbed on a polycrystalline Ag surface as evidenced by the characteristic O - O stretching frequencies in the EEL spectra. Based on temperature-variation studies of the vibration bands in the EELS and of the O(ls) core level peaks in the XPS, characteristic O(ls) binding energies are assigned to the two molecular species; the superoxo species is associated with a significantly higher binding energy as expected. The superoxo species appears to be relatively less thermally stable than the peroxo species, being associated with a - 1 3 0 0 cm -I stretching vibration, the highest O - O stretching frequency observed so far due to a chemisorbed species.
Although oxygen adsorbed on Ag and other metal surfaces has been investigated extensively in the last five years by employing electron spectroscopy and related techniques [1-4], the exact nature of the adsorbed species as well as the associated spectroscopic characteristics such as the O(ls) binding energies are not well established. While the presence of the parallelly adsorbed peroxo-type (O~-) species seems to be established on Ag and other metal surfaces, a superoxo-type (O~-) species has not been clearly identified. Since the superoxo and the peroxo species are associated with significantly different O - O bond orders and hence stretching frequencies, an EELS study would allow clear identification of the two species. The superoxo and peroxo species adsorbed on metal surfaces have been reported [3-5], but their relative thermal stabilities and spectroscopic features are not unequivocally established. We have therefore carried out a combined E E L S - X P S study of 0 2 adsorbed on an Ag surface in order to examine the different species formed and to assign the O(ls) binding energies to specific species identified by EELS. We chose a polycrystalline Ag surface for our study hoping that it would favour the occurrence of both the peroxo and the superoxo species; further* Contribution No. 425 from the Solid State and Structural Chemistry Unit. * * To whom all correspondence should be addressed.
0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L576
K. Prabhakaran, C.N.R. Rao / E E L S - X P S study of oxygen on silver
more, such a surface would be closer to that encountered in the silver catalysts employed in oxidation reactions. XPS and EELS measurements were performed with a VG ESCALAB V-EELS spectrometer. An independent set of XPS measurements was obtained with a VG ESCA 3 Mark II spectrometer. Details of the instruments are given elsewhere [2,6]. A high-purity polycrystalline foil of Ag cleaned by Ar ÷ b o m b a r d m e n t was used for the study. Oxygen was prepared by the thermal decomposition of KMnOa; the purity of the gas was checked in situ with a quadrupole mass spectrometer. The instrument was baked at 470 K before starting the experiments. X-ray photoelectron spectra were recorded using A 1 K a radiation. The exposures (made at 10 5-10 4 Torr) are expressed in langmuirs (1 L = 1 0 - 6 Tort s). In the EELS experiments, the primary energy of the electrons was 3.5 eV with a half-width of 15 meV for the elastic beam. In fig. 1 we show the EEL spectra of 02 adsorbed on the Ag surface at 80 K and then warmed to different temperatures. The spectrum shows features at 1320, 805, 645 and 270 cm -1, the last arising from the metal-oxygen stretch-
230K
01,',
&
Loss,?, ....
-
/
\
- ---
'-~-~-~.--; 230 K
~
~
C ea
[
,
1000 Energy
Ag
I 2000
L o s s ( c m -I)
Fig. 1. E E L S of 0 2 a d s o r b e d o n a n A g surface. Inset s h o w s a set o f E E L s p e c t r a o b t a i n e d o n a n o t h e r p o l y c r y s t a l l i n e A g surface.
K. Prabhakaran, C.N.R. Rao / E E L S - X P S study of o.,b,gen on sih,er
06
L577
D 1320cm I []
03
o
645cm ~
"
8 0 5 c rn~
n
01
o
700
o o
;o
2i I~
•
o
•
•
80
120
1J04
J
180
o
i ~
220
r
260
T,K Fig. 2. Variation~with temperature of the intensities of the O - O stretching bands in the EEL spectra. Notice that the intensity of the 1320 c m - ~ band goes to zero before 200 K; the other two O - O stretching bands disappear at - 230 K.
ing mode. The 1320 cm -1 band cannot be due to CO 2 or CO 2 , but can only arise from an O - O stretching vibrational mode of a molecular oxygen species. Since this stretching frequency corresponds to an O - O bond order of - 1.5 [2], it is assigned to a superoxo species. A band around 1040 cm -1 has been assigned to the superoxo species on P d ( l l l ) surface [5]: this would have bond order less than 1.5. It is possible that the 1320 cm -1 band is due to an 0 2 species inclined at an angle to the metal surface. Molecular oxygen present in the superoxo form in transition metal complexes is known to exhibit an O - O stretching band in the 1070-1200 cm -1 region [7]. The 1320 cm-~ band decreases in intensity on warming and disappears well below 200 K (fig. 2); the relatively low temperature at which the band disappears also precludes the presence of the carbonate species. The intensities of the 645 and 805 cm -1 bands decrease with increase in temperature and then disappear around 230 K (fig. 2). We assign the peaks at 645 and 805 c m - t to peroxo species with O - O bond orders of unity or less. Two similar bands have been observed when oxygen is adsorbed on a polycrystalline copper surface [2] and also on a P d ( l l l ) surface [5]. The two bands due to peroxo species appear possibly because the polycrystalline surface contains a high proportion of (111) faces, besides (110) and other faces. The observation of the superoxo species in the present study may also have something to do with this factor. The occurrence of several molecularly adsorbed oxygen species can be rationalized in terms of a multiple-well potential energy surface [3,51.
L578
K. Prabhakaran C.N.R. Rao / E E L S - X P S study of oxygen on silver
531 4 .I. I 5325 •
.. :/!/i:i~. ''
•
300K
180K
200K
120K 120K
80K BOK
,~.~.-~".~.-~-
I 522
I
[ 526
f
I 530 BE/eV
" ~
J
t 534
-
p
Clean
I 538
Ag
- ~ - -"-~U--'Y;-S--2-~-- ~=~-~,..-,~4~-,", t I 525
1 _ ~
I I 530
I
~L~'
I I 535
C lea n A g ~
I
BE/eV
Fig. 3. Lefthand side: O(ls) spectra of 02 adsorbed on an Ag surface recorded under the same conditions as the EELS in fig. 1. Right-hand side: O(ls) spectra of 02 absorbed on an Ag surface recorded independently with another spectrometer. Inset shows the difference O(ls) spectrum of the adlayer (spectrum at 80 K minus the one after warming it to 298 K) indicating the presence of two peaks•
In fig. 3, we have shown the O(ls) spectra of 0 2 adsorbed on the Ag surface recorded along with the EEL spectra in fig. 1 under the same conditions. We see two broad features around 532 and 530 eV. We have plotted the temperature variation of the intensity of the broad 532 eV feature in fig. 4; this feature disappears below 200 K just as the 1320 cm -1 band due to the superoxo species in the EEL spectra. We therefore assign the peak around 532 eV to the superoxo species. In fig. 4, we also show the temperature variation of the intensity of the 532 eV peak recorded in an independent set of measurements carried out with another spectrometer. Here again, the 532 eV feature disappears below 200 K. The broad 530 eV feature in the O(ls) spectra in fig. 3 could arise from the peroxo species a n d / o r an atomic oxygen species. We have plotted the F W H M of the 530 eV peak against temperature in fig. 4. We see that the F W H M remains nearly constant upto - 230 K and above this temperature decreases to a constant value. The F W H M above - 2 3 0 K is close to that of the Ag(3d 5/2) peak; under these conditions, the O(ls) binding energy is also lower (529.4-t-0.4 eV). We suggest that this variation in the F W H M of the 530 eV
K. Prabhakaran, C.N.R. Rao / E E L S - X P S study of oxygen on siloer
_~ o
L579
(a)
o
80
140
200 T,I,{
I
260
2l
320
(b)
:E
n I~L~-
I 80
I 140
~
I 200
i
I 260
,
I 320
T,K Fig. 4. (a) Variation with temperature of the intensity (height) of the broad O(1s) peak around 532 eV: squares are data obtained from the same adlayers as the experiment in EELS (fig. 2); circles are an independent set of data obtained with an ESCA 3 Mark II spectrometer. Notice that the intensity of the 532 eV feature goes to zero below 200 K. (b) Variation with temperature of the F W H M of the broad - 530 eV O(ls) peak. Squares and circles represent the same sets of data as in (a). Notice a drop in the F W H M above 230 K. The F W H M above 230 K is close to that of the Ag(3ds/2) peak. The O(ls) binding energy above ~ 230 K is lower (529.4_+ 0.4 eV) compared to the binding energy at lower temperatures (530.4 _+0.4 eV).
O(ls) feature arises because of the disappearance of the peroxo species around 230 K (as also revealed by EELS in figs. 1 and 2). The species on the surface above 230 K would be atomic in nature and is therefore associated with a low O(ls) binding energy (529.4 _4-0.4 eV). As mentioned earlier, the 532 eV feature in the O(ls) spectrum is broad. The difference spectrum after the adlayer at 80 K was heated to 298 K shows two discernible peaks around 531.4 and 532.5 eV (fig. 3). Two such O(1s) binding energies separated by - 1.2 eV are expected for the superoxo species as discussed by Campbell [4]. The O(ls) binding energies of the superoxo species are in the right range, being much higher than for the peroxo species on the Ag surface studied by us (530.4 + 0.4 eV). It may be remarked here that the O(ls) binding energies of the peroxo and the atomic oxygen species absorbed on transition metal surface as reported in the literature differ very
L580
K. Prabhakaran, C.N.R. Rao / E E L S - X P S
study of oxygen on sih,er
widely. Only part of this may arise from differences in calibration; it is still not clear whether the wide variation in the O(ls) binding energy of the peroxo species (529-533 eV) is truly an intrinsic feature. The thermal stability of the adsorbed superoxo species as found in the present study is lower than that for the peroxo species. If these species leave the metal surface by desorption, the relative thermal stability found by us can be understood. The peroxo species is certainly bound more strongly to the surface than the superoxo species. Such a superoxo species could indeed play a crucial role in the oxidation of olefins [8]. Studies on O 2 adsorbed on Ag (111) surfaces are now in progress. The authors thank the Department of Science and Technology, Government of India, for support of this research.
References [1] [2] [3] [4] [5] [6]
P.V. Kamath and C.N.R. Rao, J. Phys. Chem. 88 (1984) 464. K. Prabhakaran, P. Sen and C.N.R. Rao, Surface Sci. 177 (1986) L971. C.T. Campbell, Surface Sci. 157 (1985) 43. C.T. Campbell, Surface Sci. 173 (1986) E641. R. lmbihl and J.E. Demuth, Surface Sci. 173 (1986) 395. P. Sen and C.N.R. Rao, Surface Sci. 172 (1986) 269; G. Ranga Rao, K. Prabhakaran and C.N.R. Rao, Surface Sci. 176 (1986) L835. [7] L. Vaska, Acc. Chem. Res. 9 (1976) 175. [8] W.H.M. Sachtler, Catalysis Rev. 4 (1970) 27.
L575
S U R F A C E SCIENCE LETTERS A C O M B I N E D E E L S - X P S S T U D Y OF MOLECULARLY C H E M I S O R B E D O X Y G E N O N SILVER SURFACES: EVIDENCE FOR SUPEROXO AND PEROXO SPECIES * K. P R A B H A K A R A N and C.N.R. RAO * * Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India Received 6 March 1987; accepted for publication 22 April 1987
Both superoxo and peroxo species are formed when oxygen is adsorbed on a polycrystalline Ag surface as evidenced by the characteristic O - O stretching frequencies in the EEL spectra. Based on temperature-variation studies of the vibration bands in the EELS and of the O(ls) core level peaks in the XPS, characteristic O(ls) binding energies are assigned to the two molecular species; the superoxo species is associated with a significantly higher binding energy as expected. The superoxo species appears to be relatively less thermally stable than the peroxo species, being associated with a - 1 3 0 0 cm -I stretching vibration, the highest O - O stretching frequency observed so far due to a chemisorbed species.
Although oxygen adsorbed on Ag and other metal surfaces has been investigated extensively in the last five years by employing electron spectroscopy and related techniques [1-4], the exact nature of the adsorbed species as well as the associated spectroscopic characteristics such as the O(ls) binding energies are not well established. While the presence of the parallelly adsorbed peroxo-type (O~-) species seems to be established on Ag and other metal surfaces, a superoxo-type (O~-) species has not been clearly identified. Since the superoxo and the peroxo species are associated with significantly different O - O bond orders and hence stretching frequencies, an EELS study would allow clear identification of the two species. The superoxo and peroxo species adsorbed on metal surfaces have been reported [3-5], but their relative thermal stabilities and spectroscopic features are not unequivocally established. We have therefore carried out a combined E E L S - X P S study of 0 2 adsorbed on an Ag surface in order to examine the different species formed and to assign the O(ls) binding energies to specific species identified by EELS. We chose a polycrystalline Ag surface for our study hoping that it would favour the occurrence of both the peroxo and the superoxo species; further* Contribution No. 425 from the Solid State and Structural Chemistry Unit. * * To whom all correspondence should be addressed.
0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L576
K. Prabhakaran, C.N.R. Rao / E E L S - X P S study of oxygen on silver
more, such a surface would be closer to that encountered in the silver catalysts employed in oxidation reactions. XPS and EELS measurements were performed with a VG ESCALAB V-EELS spectrometer. An independent set of XPS measurements was obtained with a VG ESCA 3 Mark II spectrometer. Details of the instruments are given elsewhere [2,6]. A high-purity polycrystalline foil of Ag cleaned by Ar ÷ b o m b a r d m e n t was used for the study. Oxygen was prepared by the thermal decomposition of KMnOa; the purity of the gas was checked in situ with a quadrupole mass spectrometer. The instrument was baked at 470 K before starting the experiments. X-ray photoelectron spectra were recorded using A 1 K a radiation. The exposures (made at 10 5-10 4 Torr) are expressed in langmuirs (1 L = 1 0 - 6 Tort s). In the EELS experiments, the primary energy of the electrons was 3.5 eV with a half-width of 15 meV for the elastic beam. In fig. 1 we show the EEL spectra of 02 adsorbed on the Ag surface at 80 K and then warmed to different temperatures. The spectrum shows features at 1320, 805, 645 and 270 cm -1, the last arising from the metal-oxygen stretch-
230K
01,',
&
Loss,?, ....
-
/
\
- ---
'-~-~-~.--; 230 K
~
~
C ea
[
,
1000 Energy
Ag
I 2000
L o s s ( c m -I)
Fig. 1. E E L S of 0 2 a d s o r b e d o n a n A g surface. Inset s h o w s a set o f E E L s p e c t r a o b t a i n e d o n a n o t h e r p o l y c r y s t a l l i n e A g surface.
K. Prabhakaran, C.N.R. Rao / E E L S - X P S study of o.,b,gen on sih,er
06
L577
D 1320cm I []
03
o
645cm ~
"
8 0 5 c rn~
n
01
o
700
o o
;o
2i I~
•
o
•
•
80
120
1J04
J
180
o
i ~
220
r
260
T,K Fig. 2. Variation~with temperature of the intensities of the O - O stretching bands in the EEL spectra. Notice that the intensity of the 1320 c m - ~ band goes to zero before 200 K; the other two O - O stretching bands disappear at - 230 K.
ing mode. The 1320 cm -1 band cannot be due to CO 2 or CO 2 , but can only arise from an O - O stretching vibrational mode of a molecular oxygen species. Since this stretching frequency corresponds to an O - O bond order of - 1.5 [2], it is assigned to a superoxo species. A band around 1040 cm -1 has been assigned to the superoxo species on P d ( l l l ) surface [5]: this would have bond order less than 1.5. It is possible that the 1320 cm -1 band is due to an 0 2 species inclined at an angle to the metal surface. Molecular oxygen present in the superoxo form in transition metal complexes is known to exhibit an O - O stretching band in the 1070-1200 cm -1 region [7]. The 1320 cm-~ band decreases in intensity on warming and disappears well below 200 K (fig. 2); the relatively low temperature at which the band disappears also precludes the presence of the carbonate species. The intensities of the 645 and 805 cm -1 bands decrease with increase in temperature and then disappear around 230 K (fig. 2). We assign the peaks at 645 and 805 c m - t to peroxo species with O - O bond orders of unity or less. Two similar bands have been observed when oxygen is adsorbed on a polycrystalline copper surface [2] and also on a P d ( l l l ) surface [5]. The two bands due to peroxo species appear possibly because the polycrystalline surface contains a high proportion of (111) faces, besides (110) and other faces. The observation of the superoxo species in the present study may also have something to do with this factor. The occurrence of several molecularly adsorbed oxygen species can be rationalized in terms of a multiple-well potential energy surface [3,51.
L578
K. Prabhakaran C.N.R. Rao / E E L S - X P S study of oxygen on silver
531 4 .I. I 5325 •
.. :/!/i:i~. ''
•
300K
180K
200K
120K 120K
80K BOK
,~.~.-~".~.-~-
I 522
I
[ 526
f
I 530 BE/eV
" ~
J
t 534
-
p
Clean
I 538
Ag
- ~ - -"-~U--'Y;-S--2-~-- ~=~-~,..-,~4~-,", t I 525
1 _ ~
I I 530
I
~L~'
I I 535
C lea n A g ~
I
BE/eV
Fig. 3. Lefthand side: O(ls) spectra of 02 adsorbed on an Ag surface recorded under the same conditions as the EELS in fig. 1. Right-hand side: O(ls) spectra of 02 absorbed on an Ag surface recorded independently with another spectrometer. Inset shows the difference O(ls) spectrum of the adlayer (spectrum at 80 K minus the one after warming it to 298 K) indicating the presence of two peaks•
In fig. 3, we have shown the O(ls) spectra of 0 2 adsorbed on the Ag surface recorded along with the EEL spectra in fig. 1 under the same conditions. We see two broad features around 532 and 530 eV. We have plotted the temperature variation of the intensity of the broad 532 eV feature in fig. 4; this feature disappears below 200 K just as the 1320 cm -1 band due to the superoxo species in the EEL spectra. We therefore assign the peak around 532 eV to the superoxo species. In fig. 4, we also show the temperature variation of the intensity of the 532 eV peak recorded in an independent set of measurements carried out with another spectrometer. Here again, the 532 eV feature disappears below 200 K. The broad 530 eV feature in the O(ls) spectra in fig. 3 could arise from the peroxo species a n d / o r an atomic oxygen species. We have plotted the F W H M of the 530 eV peak against temperature in fig. 4. We see that the F W H M remains nearly constant upto - 230 K and above this temperature decreases to a constant value. The F W H M above - 2 3 0 K is close to that of the Ag(3d 5/2) peak; under these conditions, the O(ls) binding energy is also lower (529.4-t-0.4 eV). We suggest that this variation in the F W H M of the 530 eV
K. Prabhakaran, C.N.R. Rao / E E L S - X P S study of oxygen on siloer
_~ o
L579
(a)
o
80
140
200 T,I,{
I
260
2l
320
(b)
:E
n I~L~-
I 80
I 140
~
I 200
i
I 260
,
I 320
T,K Fig. 4. (a) Variation with temperature of the intensity (height) of the broad O(1s) peak around 532 eV: squares are data obtained from the same adlayers as the experiment in EELS (fig. 2); circles are an independent set of data obtained with an ESCA 3 Mark II spectrometer. Notice that the intensity of the 532 eV feature goes to zero below 200 K. (b) Variation with temperature of the F W H M of the broad - 530 eV O(ls) peak. Squares and circles represent the same sets of data as in (a). Notice a drop in the F W H M above 230 K. The F W H M above 230 K is close to that of the Ag(3ds/2) peak. The O(ls) binding energy above ~ 230 K is lower (529.4_+ 0.4 eV) compared to the binding energy at lower temperatures (530.4 _+0.4 eV).
O(ls) feature arises because of the disappearance of the peroxo species around 230 K (as also revealed by EELS in figs. 1 and 2). The species on the surface above 230 K would be atomic in nature and is therefore associated with a low O(ls) binding energy (529.4 _4-0.4 eV). As mentioned earlier, the 532 eV feature in the O(ls) spectrum is broad. The difference spectrum after the adlayer at 80 K was heated to 298 K shows two discernible peaks around 531.4 and 532.5 eV (fig. 3). Two such O(1s) binding energies separated by - 1.2 eV are expected for the superoxo species as discussed by Campbell [4]. The O(ls) binding energies of the superoxo species are in the right range, being much higher than for the peroxo species on the Ag surface studied by us (530.4 + 0.4 eV). It may be remarked here that the O(ls) binding energies of the peroxo and the atomic oxygen species absorbed on transition metal surface as reported in the literature differ very
L580
K. Prabhakaran, C.N.R. Rao / E E L S - X P S
study of oxygen on sih,er
widely. Only part of this may arise from differences in calibration; it is still not clear whether the wide variation in the O(ls) binding energy of the peroxo species (529-533 eV) is truly an intrinsic feature. The thermal stability of the adsorbed superoxo species as found in the present study is lower than that for the peroxo species. If these species leave the metal surface by desorption, the relative thermal stability found by us can be understood. The peroxo species is certainly bound more strongly to the surface than the superoxo species. Such a superoxo species could indeed play a crucial role in the oxidation of olefins [8]. Studies on O 2 adsorbed on Ag (111) surfaces are now in progress. The authors thank the Department of Science and Technology, Government of India, for support of this research.
References [1] [2] [3] [4] [5] [6]
P.V. Kamath and C.N.R. Rao, J. Phys. Chem. 88 (1984) 464. K. Prabhakaran, P. Sen and C.N.R. Rao, Surface Sci. 177 (1986) L971. C.T. Campbell, Surface Sci. 157 (1985) 43. C.T. Campbell, Surface Sci. 173 (1986) E641. R. lmbihl and J.E. Demuth, Surface Sci. 173 (1986) 395. P. Sen and C.N.R. Rao, Surface Sci. 172 (1986) 269; G. Ranga Rao, K. Prabhakaran and C.N.R. Rao, Surface Sci. 176 (1986) L835. [7] L. Vaska, Acc. Chem. Res. 9 (1976) 175. [8] W.H.M. Sachtler, Catalysis Rev. 4 (1970) 27.