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A study of NO adsorbed on alumina-supported Rh by IETS
Surface Science 114( 1982) L21 -L27 North-Holland Publishing Company
SURFACE
SCIENCE
L2l
LETTERS
A STUDY OF NO ADSORBED IETS
ON ALUMINA-SUPPORTED
Rh BY
S. DE CHEVEIGNk,
S. GAUTHIER and J. KLEIN Groupe de Ph,vsique des Solides de I’ENS, UninersitP Paris VII, Tour 23. 2 Piwe Jussieu, F-75005 Pork
Cede-x 05, Frunce
Received
12 August
198 1
Inelastic Electron Tunneling Spectroscopy has been used to study the adsorption of NO on evaporated Rh particles supported on alumina. The molecule adsorbs dissociatively and the nitrogen reacts with hydroxyl groups on the alumina surface to form a RhNH, species, as shown alumina surface. by studies using 15N0 and a deuteroxyled
Rhodium is a widely used catalyst for the reduction of nitric oxide, in particular in anti-pollution devices [l]. This reduction often takes place in the presence of carbon monoxide. It may produce N,, or NH, if water vapour is present, N, being obviously less polluting. To understand the mechanisms of these reactions a number of adsorption studies have been undertakz. Infrared spectroscopy has been used to study the adsorption of NO on alumina supported Rh [2,3]. N-O elongation modes of various configurations were observed indicating the presence of undissociated NO, but the low energy vibrations of the metal-molecule band are not visible in IR spectroscopy due to absorption by the supporting oxide. LEED and TDS have brought more information [4,5]. NO has been shown to dissociate on monocrystalline Rh at low exposures (below 0.5 to 1 L), and subsequently to adsorb associatively at higher exposures. The coadsorption of NO and CO has also been studied by infrared spectroscopy [l-3]. An isocyanate species Rh-NC0 was observed, which is thought to react with water vapour to form NH,. In the preliminary study reported here we have used Inelastic Electron Tunneling Spectroscopy to try to bring additional information on these adsorption mechanisms. Tunneling spectroscopy has been described elsewhere [6,7]. The main advantages for this type of study are its resolution (< 4 cm-‘) and its frequency range (200-4000 cm-‘). Tunnel junctions are formed by oxidizing an aluminum electrode. A small quantity of Rh is evaporated onto the oxide under a pressure of 5 X 10 -5 Torr of NO. The average Rh thickness is measured by a quartz microbalance; the metal forms islands of about 25 A diameter [8]. The 0039-6028/82/0000-0000,/$02.75
0 1982 North-Holland
0
200
loo
I. Tunneling curve).
Fig.
spectra
of an undoped
300
400
V(meV)
sample (lower curve) and of NO adsorbed
on Rh
(upper
bell jar is then evacuated and a lead counterelectrode is deposited. When a bias is applied, electrons tunnel through the junction and can excite vibrations of the molecules. This shows up as a variation in the junction conductance, or as a peak in the curve of the derivative of the conductance versus applied voltage. The result is a spectrum analogous to an IR or Raman spectrum such as shown in fig. 1. The junctions are made at room temperature but the spectra are taken at 4.2 K or less to reduce thermal broadening. One could ask about the presence of a top metal electrode on the adsorbed molecule. Experimental studies [9- 1 I] have shown that, surprisingly enough, this counterelectrode has generally little influence on the position of the peak (less than 1%). However, it seems to be more dramatic in the few cases where the dipole derivative of the considered mode is particularly strong. These energy shifts can be explained by the interaction of the dipole of the molecule with its image in the electrode [9]. It is to be noted that some residual CO is present during the Rh evaporation - it appears on the spectrum of the metal deposited with no gas introduced. We estimate the partial pressure of CO during the Rh evaporation to be of the order of 10 -’ Torr. Nevertheless, no trace of CO appears in the spectra of NO. Fig. 1 shows the spectrum obtained for NO adsorbed on Rh and fig. 2 shows how it varies with the average thickness of the metal. The main feature is a peak at about 57 meV (460 cm-‘) that increases in intensity and decreases in frequency (fig.4) as the quantity of Rh increases. Two shoulders appear at about 30 and 40 meV (240 and 320 cm-‘). The second is the overshoot of a feature due to electrode phonons, but the peak at 30 meV may be significative, although its position is difficult to determine ‘on the rapidly varying background.
S. de Checeig&
_-1 do 0 dU
et al. ,j NO
on
alumiilu-supported
Rh
L23
Al_Ox_Rh_NO_Pb
Fig. 2. Tunneling spectra is an average (see text).
of NO adsorbed
on incrcaslng
quantities
of Rh. The thickness
Indicated
n
coo
450
500
V(meV)
Fig. 3. High energy region of the spectra of an undoped junction, of a junction with CO adsorbed on Rh, and of a junction with NO adsorbed on Rh. Only in the last case is the OH elongation removed. A new peak also appears at about 405 meV (3200 cmp’ ).
although its position is difficult to determine on the rapidly varying background. Another weak peak appears at 404 * 2 meV (3260 * 15 cm-‘). The OH elongation usually present on tunnel spectra (fig. l), due to hydroxyl radicals on the alumina surface, has disappeared. This effect is not observed with CO on Rh, as can be seen in fig. 3. No peaks are observed in the 170-250 meV (1400-2000 cm- ‘) region where N-O or C-O elongations are expected. To help identify these features we have used “NO. The main low energy peak is substantially shifted (-4.4 t 0.8%). These shifts must be determined at a given average thickness of Rh since the peak position varies. To obtain more precision we have used as a measure of the quantity of NO the ratio of the area of the 460 cm-’ peak to that of the Al-O feature (fig. 4). The isotopic shift is seen to be practically independent of the quantity of Rh. On the contrary the high energy mode at 3300 cm-’ is unaffected to within our resolution. The shift, if it exists, is less than 0.5%. We also studied junctions with a barrier made in a glow discharge in D,O. The result is an oxide with OD radicals on the surface. When NO is adsorbed on Rh (fig. 5), the OD elongation peak disappears, as did voH and a new peak appears at about 295 meV (2380 cm-‘). The region below 800 cm-’ is also affected, but in a complex manner because the NO modes are now superim-
52
54
56
50
60
v(meV)
Fig. 4. Measured peak positions versus relative intensity of the low energy peak for 14N0 and 15N0. Absolute pos’ItIons are obtained by subtracting the shift due to the gap of the superconducting lead electrode (0.8 meV).
S. de Cheveigtk
Al_Ox(
et ul. /
300
L25
Rh
D20) _ Rh_Pb
AI_Ox(DpO)_
250
NO on ulumina-supported
350
g. 5. High energy region of the spectrum elongation is removed and a peak appears
Rh_NO_Pb
V
( mcV)
of NO@ on a deuteroxyled at 295 meV (2380 cm- ‘).
alumina
surface.
The OD
posed on bending modes of the OD radicals in the bulk of the oxide (fig. 6). Finally we checked that NO did not adsorb on junctions with no Rh deposited. The absence of NO elogation modes expected in the 170-250 meV (14002000 cm-‘) region points to a dissociation of the molecule on the Rh particles. The peak near 57 meV (460 cm-‘) could therefore be a R&N or a m-0
V(rn&)
100 Fig.
6. Tunneling
spectrum
200
of NO adsorbed
300
LOO
on Rh deposited
on a deutcroxyled
alumina
surface.
species. The shift observed when “N is substituted in the NO molecule is strong, as expected for uRh_N (the ratio of the roots of the reduced masses predicts a shift of 3.5% and we observed 4.4 + 0.8%). This is much more than would be expected for any mode of an undissociated RI-NO species, and of course a Rl-0 mode should not shift at all. We did not observe Rh-0 expected at 65 meV (520 cm-‘) [ 121, but this is consistent with the fact that we have never been able to observe any mode of this type when 0, adsorbs on Rh. The 0 may diffuse into the Rh as has been observed at higher temperature [ 131, or it may react immediately with the residual CO present in the bell jar to form CO, [14] which would then desorb. to an N-H elongation. It The peak near 3300 cm-’ can be attributed should be barely sensitive to “N substitut i o n ( th e root of the ratio of reduced masses predicts a shift of only 0.25%) which is consistent with our observations. On the other hand D substitution should produce a shift of about l/D and we found a value of about l/1.37. Unfortunately, our spectra are not sufficiently intense to allow a precise identification of the NH, species; we cannot, for example, clearly distinguish any deformation mode. The hydrogen or deuterium, necessary to form these species, clearly comes from the hydroxyl groups on the alumina surface since their elongation mode disappears. So far, we cannot affirm that all the N atoms have formed these NH, species nor that these are on the Rh rather than on the alumina, but it should be noted that: (a) the presence of one to three H atoms would not substantially affect the “N isotopic shifts of ~a~_~; (b) the fact that the Rh-N elongations appear far more intense than the N-H elongations is not necessarily significative. The same abnormal intensity of metal-carbon modes to C-O elongations was observed with CO adsorbed on Rh [7,8], and has not yet been satisfactorily explained. Further studies are underway to try to elucidate the reaction mechanism between NO, Rh and the OH radicals, and to identify the products more precisely. We hope at the same time to resolve the problem of the intensity ratios [ 151.
References [I] [2] [3] [4] [5] [6] [7]
M.L. Unland, Science 179 (1973) 567. H. Arai and H. Tominaya, .I. Catalysis 43 (1976) 131. F. Solymosi and J. Sarkany, Appl. Surface Sci. 3 (1979) 6X. R.J. Baird, R.C. Ku and P. Wynblatt. Surface Sci. 97 (1980) 346. D.G. Castner and G.A. Somorjai, Surface Sci. 83 (1979) 60. P.K. Hansma, Phys. Rept. 30C (1977) 145. J. Klein, A. L@er, S. de Cheveigne. C. Guinet. M. Belin and D. Defourneau, (I 979) L288. [8] R.M. Krocker, W.C. Kaska and P.K. Hansma, J. Catalysis 57 (1979) 72. [U] J.R. Kirtley and P.K. Hansma, Phys. Rev. Bl2 (1975) 531.
Surface
SCI. X2
S. de CheveignP er ul. / NO on aluminu-supported
[IO] [I I] [ 121 [I31 [ 141 [ 151
Rh
J.R. Kirtley and P.K. Hansma, Phys. Rev. Bl3 (1976) 2910. A. Bayman, P.K. Hansma and W.C. Kuska. Phys. Rev. B25 (1981) 2449 L.H. Dubois and G.A. Somorjai, Surface Sci. 9 I (I 980) 5 14. Y. Kim, SK. Shi and J.M. White, J. Catalysis 61 (1980) 374. C.T. Campbell and J.M. White, Appl. Surface Sci. I (1978) 347. S. Gauthier. J. Klein. A. Leger and S. de Cheveign& to be published.
L27
SURFACE
SCIENCE
L2l
LETTERS
A STUDY OF NO ADSORBED IETS
ON ALUMINA-SUPPORTED
Rh BY
S. DE CHEVEIGNk,
S. GAUTHIER and J. KLEIN Groupe de Ph,vsique des Solides de I’ENS, UninersitP Paris VII, Tour 23. 2 Piwe Jussieu, F-75005 Pork
Cede-x 05, Frunce
Received
12 August
198 1
Inelastic Electron Tunneling Spectroscopy has been used to study the adsorption of NO on evaporated Rh particles supported on alumina. The molecule adsorbs dissociatively and the nitrogen reacts with hydroxyl groups on the alumina surface to form a RhNH, species, as shown alumina surface. by studies using 15N0 and a deuteroxyled
Rhodium is a widely used catalyst for the reduction of nitric oxide, in particular in anti-pollution devices [l]. This reduction often takes place in the presence of carbon monoxide. It may produce N,, or NH, if water vapour is present, N, being obviously less polluting. To understand the mechanisms of these reactions a number of adsorption studies have been undertakz. Infrared spectroscopy has been used to study the adsorption of NO on alumina supported Rh [2,3]. N-O elongation modes of various configurations were observed indicating the presence of undissociated NO, but the low energy vibrations of the metal-molecule band are not visible in IR spectroscopy due to absorption by the supporting oxide. LEED and TDS have brought more information [4,5]. NO has been shown to dissociate on monocrystalline Rh at low exposures (below 0.5 to 1 L), and subsequently to adsorb associatively at higher exposures. The coadsorption of NO and CO has also been studied by infrared spectroscopy [l-3]. An isocyanate species Rh-NC0 was observed, which is thought to react with water vapour to form NH,. In the preliminary study reported here we have used Inelastic Electron Tunneling Spectroscopy to try to bring additional information on these adsorption mechanisms. Tunneling spectroscopy has been described elsewhere [6,7]. The main advantages for this type of study are its resolution (< 4 cm-‘) and its frequency range (200-4000 cm-‘). Tunnel junctions are formed by oxidizing an aluminum electrode. A small quantity of Rh is evaporated onto the oxide under a pressure of 5 X 10 -5 Torr of NO. The average Rh thickness is measured by a quartz microbalance; the metal forms islands of about 25 A diameter [8]. The 0039-6028/82/0000-0000,/$02.75
0 1982 North-Holland
0
200
loo
I. Tunneling curve).
Fig.
spectra
of an undoped
300
400
V(meV)
sample (lower curve) and of NO adsorbed
on Rh
(upper
bell jar is then evacuated and a lead counterelectrode is deposited. When a bias is applied, electrons tunnel through the junction and can excite vibrations of the molecules. This shows up as a variation in the junction conductance, or as a peak in the curve of the derivative of the conductance versus applied voltage. The result is a spectrum analogous to an IR or Raman spectrum such as shown in fig. 1. The junctions are made at room temperature but the spectra are taken at 4.2 K or less to reduce thermal broadening. One could ask about the presence of a top metal electrode on the adsorbed molecule. Experimental studies [9- 1 I] have shown that, surprisingly enough, this counterelectrode has generally little influence on the position of the peak (less than 1%). However, it seems to be more dramatic in the few cases where the dipole derivative of the considered mode is particularly strong. These energy shifts can be explained by the interaction of the dipole of the molecule with its image in the electrode [9]. It is to be noted that some residual CO is present during the Rh evaporation - it appears on the spectrum of the metal deposited with no gas introduced. We estimate the partial pressure of CO during the Rh evaporation to be of the order of 10 -’ Torr. Nevertheless, no trace of CO appears in the spectra of NO. Fig. 1 shows the spectrum obtained for NO adsorbed on Rh and fig. 2 shows how it varies with the average thickness of the metal. The main feature is a peak at about 57 meV (460 cm-‘) that increases in intensity and decreases in frequency (fig.4) as the quantity of Rh increases. Two shoulders appear at about 30 and 40 meV (240 and 320 cm-‘). The second is the overshoot of a feature due to electrode phonons, but the peak at 30 meV may be significative, although its position is difficult to determine ‘on the rapidly varying background.
S. de Checeig&
_-1 do 0 dU
et al. ,j NO
on
alumiilu-supported
Rh
L23
Al_Ox_Rh_NO_Pb
Fig. 2. Tunneling spectra is an average (see text).
of NO adsorbed
on incrcaslng
quantities
of Rh. The thickness
Indicated
n
coo
450
500
V(meV)
Fig. 3. High energy region of the spectra of an undoped junction, of a junction with CO adsorbed on Rh, and of a junction with NO adsorbed on Rh. Only in the last case is the OH elongation removed. A new peak also appears at about 405 meV (3200 cmp’ ).
although its position is difficult to determine on the rapidly varying background. Another weak peak appears at 404 * 2 meV (3260 * 15 cm-‘). The OH elongation usually present on tunnel spectra (fig. l), due to hydroxyl radicals on the alumina surface, has disappeared. This effect is not observed with CO on Rh, as can be seen in fig. 3. No peaks are observed in the 170-250 meV (1400-2000 cm- ‘) region where N-O or C-O elongations are expected. To help identify these features we have used “NO. The main low energy peak is substantially shifted (-4.4 t 0.8%). These shifts must be determined at a given average thickness of Rh since the peak position varies. To obtain more precision we have used as a measure of the quantity of NO the ratio of the area of the 460 cm-’ peak to that of the Al-O feature (fig. 4). The isotopic shift is seen to be practically independent of the quantity of Rh. On the contrary the high energy mode at 3300 cm-’ is unaffected to within our resolution. The shift, if it exists, is less than 0.5%. We also studied junctions with a barrier made in a glow discharge in D,O. The result is an oxide with OD radicals on the surface. When NO is adsorbed on Rh (fig. 5), the OD elongation peak disappears, as did voH and a new peak appears at about 295 meV (2380 cm-‘). The region below 800 cm-’ is also affected, but in a complex manner because the NO modes are now superim-
52
54
56
50
60
v(meV)
Fig. 4. Measured peak positions versus relative intensity of the low energy peak for 14N0 and 15N0. Absolute pos’ItIons are obtained by subtracting the shift due to the gap of the superconducting lead electrode (0.8 meV).
S. de Cheveigtk
Al_Ox(
et ul. /
300
L25
Rh
D20) _ Rh_Pb
AI_Ox(DpO)_
250
NO on ulumina-supported
350
g. 5. High energy region of the spectrum elongation is removed and a peak appears
Rh_NO_Pb
V
( mcV)
of NO@ on a deuteroxyled at 295 meV (2380 cm- ‘).
alumina
surface.
The OD
posed on bending modes of the OD radicals in the bulk of the oxide (fig. 6). Finally we checked that NO did not adsorb on junctions with no Rh deposited. The absence of NO elogation modes expected in the 170-250 meV (14002000 cm-‘) region points to a dissociation of the molecule on the Rh particles. The peak near 57 meV (460 cm-‘) could therefore be a R&N or a m-0
V(rn&)
100 Fig.
6. Tunneling
spectrum
200
of NO adsorbed
300
LOO
on Rh deposited
on a deutcroxyled
alumina
surface.
species. The shift observed when “N is substituted in the NO molecule is strong, as expected for uRh_N (the ratio of the roots of the reduced masses predicts a shift of 3.5% and we observed 4.4 + 0.8%). This is much more than would be expected for any mode of an undissociated RI-NO species, and of course a Rl-0 mode should not shift at all. We did not observe Rh-0 expected at 65 meV (520 cm-‘) [ 121, but this is consistent with the fact that we have never been able to observe any mode of this type when 0, adsorbs on Rh. The 0 may diffuse into the Rh as has been observed at higher temperature [ 131, or it may react immediately with the residual CO present in the bell jar to form CO, [14] which would then desorb. to an N-H elongation. It The peak near 3300 cm-’ can be attributed should be barely sensitive to “N substitut i o n ( th e root of the ratio of reduced masses predicts a shift of only 0.25%) which is consistent with our observations. On the other hand D substitution should produce a shift of about l/D and we found a value of about l/1.37. Unfortunately, our spectra are not sufficiently intense to allow a precise identification of the NH, species; we cannot, for example, clearly distinguish any deformation mode. The hydrogen or deuterium, necessary to form these species, clearly comes from the hydroxyl groups on the alumina surface since their elongation mode disappears. So far, we cannot affirm that all the N atoms have formed these NH, species nor that these are on the Rh rather than on the alumina, but it should be noted that: (a) the presence of one to three H atoms would not substantially affect the “N isotopic shifts of ~a~_~; (b) the fact that the Rh-N elongations appear far more intense than the N-H elongations is not necessarily significative. The same abnormal intensity of metal-carbon modes to C-O elongations was observed with CO adsorbed on Rh [7,8], and has not yet been satisfactorily explained. Further studies are underway to try to elucidate the reaction mechanism between NO, Rh and the OH radicals, and to identify the products more precisely. We hope at the same time to resolve the problem of the intensity ratios [ 151.
References [I] [2] [3] [4] [5] [6] [7]
M.L. Unland, Science 179 (1973) 567. H. Arai and H. Tominaya, .I. Catalysis 43 (1976) 131. F. Solymosi and J. Sarkany, Appl. Surface Sci. 3 (1979) 6X. R.J. Baird, R.C. Ku and P. Wynblatt. Surface Sci. 97 (1980) 346. D.G. Castner and G.A. Somorjai, Surface Sci. 83 (1979) 60. P.K. Hansma, Phys. Rept. 30C (1977) 145. J. Klein, A. L@er, S. de Cheveigne. C. Guinet. M. Belin and D. Defourneau, (I 979) L288. [8] R.M. Krocker, W.C. Kaska and P.K. Hansma, J. Catalysis 57 (1979) 72. [U] J.R. Kirtley and P.K. Hansma, Phys. Rev. Bl2 (1975) 531.
Surface
SCI. X2
S. de CheveignP er ul. / NO on aluminu-supported
[IO] [I I] [ 121 [I31 [ 141 [ 151
Rh
J.R. Kirtley and P.K. Hansma, Phys. Rev. Bl3 (1976) 2910. A. Bayman, P.K. Hansma and W.C. Kuska. Phys. Rev. B25 (1981) 2449 L.H. Dubois and G.A. Somorjai, Surface Sci. 9 I (I 980) 5 14. Y. Kim, SK. Shi and J.M. White, J. Catalysis 61 (1980) 374. C.T. Campbell and J.M. White, Appl. Surface Sci. I (1978) 347. S. Gauthier. J. Klein. A. Leger and S. de Cheveign& to be published.
L27