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Laser raman spectra of molecules adsorbed on silica surfaces
Volume 43, number 1
CHEMICAL PHYSICS LETTERS
1 Octobn
1976
LASER RAMAN SPECTRA OF MOLECULES ADSORBED ON SILICA SURFACES H. JEZIOROWSKJ and H. KNOZINCER lnstitut
ftir Physikolitche
Chemie. ilnivenit&
* Mixhen.
8 Munich 2, West Germany
Received 25 June 1976 The laser Raman spectra of chloroform, carbon tetnchloride, trans-1,2dichloroethylene, tctrachlorocthylene. tetramethytethylene. wt.-butyl cyanide, benzene and cyclohcxane adsorbed on silica surfaces have been remrded. The Raman shifts of the adsorbed molecules do not significantly dcviatc from those of the liquids and the Raman selection rules arc not viotted by the adsorption The relative intensities of the Raman bands of adsorbed species arc distinct from those of the liquids. Rusons for these intensity changes on adsorption arc discusscd.?hc surfaa electric field is found to be an important factor. .
laser Raman spectra of a variety of unsaturated molecules, which were adsorbed on the surfaces of silica or porous glass have been reported previously [ I,2 1, whereas Raman spectroscopy seems not to have been applied to studies of more weakly adsorbed molecules. such as e.g. saturated hydrocarbons. The Raman shifts of the molecules adsorbed on silicas usually very closely resembled that of the corresponding liquids, however, significant deviations of relative band intensities of adsorbed benzene [3,4], carbon tetrc chloride (5) and thiophene (61 from those in the liquid phase were observed. Analogous results were obtained by KnSzinger and Krietenbrink [7] with ferr.-butyl cyanide weakly adsorbed on v-AJ~ CJ3. This phenomenon was explained as being due to the specific H-bonding interactions between the surface hydroxyl groups and n-electron systems [4,6] and/or by the effect of the refractive index in connection with the directional nature of the adsorption interaction [6]. Bahnick and Person [8] have shown, that weak charge transfer interactions influence the relative intensrties of CCl, in solution. Such interactions may also occur on silica surfaces. However, the intensity increases reported by Bahtick and Person [8] were only by a factor of 1.6 for the strongest electron donor molecules, whereas the relative intensities of certain Raman lines of adsorbed molecules changed by factors as high as 2 to 4 [7] (see also table 1). We have therefore measured the relative intensities of various molecules adsorbed on silica, including cycloThe
and halogenated
hexane as an example of a saturated hydrocarbon w’tich cannot undergo specific interactions. tins-1,2dichIoroethylene, tetrachloroethylene. tetramethylethylene and benzene - molecules which posses a center of symmetry - were also included so that a possible violation of RIman selection rules due to the adsorption interaction must be detectable by the appearance of new Raman bands. The spectra were recorded on a Cary 82 spectrometer using the 90” optics. The greaseI_s sample tubes were similar to those described by Hendra and co-workers [9 J. The argon ion laser line at 5 14.5 nm was used as the exciting line, the laser output being set at 200 mW. ‘this laser power did not cause any thermal desoX+ion; the Raman line intensities increased linearly on laser excitation with output power up to 600 mW and started dccreasing only at higher excitation power. Standard recording conditions are given in the legend to fig. I. The silica used as adsorbent was AEROSCL 200 from Degussa with an N, BET surface area of 170 m2/g. The oxide powder was slightly pressed into the samole tube and treated in an 0, atmosphere at 500°C bef&e evacuation. This procedure very effectively eliminated serious “fluorescence** problems. The adsorptives were all p.A. grade and dried over Linde molecuhrr sieve 3A before use. They were adsorbed on the silica under their saturation vapour pressure at room temperature. The Raman spectra of liquid CC$ and of CC+ adsorbed on silica are compared in fig. 1. The positions of the bands are practically identical in both phases and the 37
Volume 43, number 1
CHEMICAL PHYSICS LETTERS
b
Fig. 1. Raman spectra of liquid C-(a) and Cf& adsorbed on silica (b). (a) Spectral slit width: 0.5 cm-‘, scan speed 1 cm-‘/s; pen period: 1 s; sensitivity 5000 counts/s; excitation: 514.5 nm. 600 mW; (b) spectral slit width: 0.75 cm-‘. scan speed: 0.1 cm?/s, pen period: 20 s, sensitivity: 200 counts/s, excitation: 514.5 nm. 200 mW.
same conclusion holds for nearly all the observed Raman bands of the adsorption systems studied. As seen from the data summarized in table 1, the maximum difference in band position is 3 cm” in the case of nuns-l,?_dichloroethylene. Another important observation is, that no IR active bands of the free molecules with center of symmetry appear in the Raman spectra of the adsorbed molecules. In these cases the principle of mutual exclusion which holds for the free molecules, might be expected to become invalid for the adsorbed species. A perturbation of the molecular symmetry must indeed be induced by the interaction with the solid surface, since IR forbidden transitions are activated and arc observed in the IR spec tra of the a&orbed molecules. E.g. strong bands are detected at 1712 and 1656 cm” on adsorption of tetramethylethylene and frrrn=-1,2dichloroethylene, respec.38
1 Octoba
1976
tively, which must be-assigned as the stretching vibration of the C=C double bond. It must therefore be concluded that neither specific nor unspecific interactions of the adsorbed molecules with the surface cause sufficiently severe distortions of the molecular symmetry as to produce significant Raman band displacements or to activate Raman inactive transitions. These observations and conclusions are in agreement, with those reported by Buechler and Turkevich [3], Egerton et al. (4) and Cooncy and co-workers [6). Fig. 1 also demonstrates that the relative intensities of the CCl, Raman bands are significantly altered in the adsorbed state as compared to the liquid. The intensity data for the other systems are summarized in table 1. These data clearly show that the various normal modes of a given molecule are affected differently by the adsorption. The most remarkable effects are observed for chloroform, for which the relative intensity of the r6 mode at 262 cm” is enhanced by a factor of 4.3. But even in the case of cyclohexane - a nonpolar molecule which should riot be able to undergo any kind of specific adsorption interaction - the relative intensity of the band at 1442 cm-t is enhanced by a factor of 2.2. Unsaturated and halogen containing hydrocarbons cause characteristic perturbations of surface hydroxyl groups when they are adsorbed on silica surfaces [lo] and these perturbations indicate specific adsorption interactions of the H-bond type. H-bonding may lead to intensity changes as suggested by Cooney and co-workers in the case of thiophene adsorption [6]. The formation of weak charge transfer complexes may be responsible for the observed phenomena with chlorine containing compounds, as proposed for CCl, in solvents of varying electron donor strength by Bahnick and Person [8]. These effects, however, cannot account for the intensity changes of theRaman bands of cyclohexane. Even in a structured liquid, the molecules “see” a nearly isotropic environment, whereas an adsorbed molecule on a solid surface is located in an extremely anisotropic environment and must be distorted and polarized by the strong surface electric fields which are known to reach values of the order of magnitude of lo6 V/cm. Aussenegg et al. [l 1) have recently reported intensity measurements of Raman lines of liquids in electric fields (0 to S x 10s V/cm). They observe significant intensity enhancements of certain Raman bands with increasing field strength and different normal modes are affected
Volume 43, nurnbcr
1
Tabie 1 Relative Raman intensities
CHEMlCAL
in liquids and in the adsorbed Liquid
Compound
AIR
cydohexane
benzene
a)
b,
chloroform
b,
carbon tetrachloridc
b)
@rms-1,2_dichlorocthylcnc
tetrahloro
ethyiene
b,
ferf.-butyl cyanide W)
b,
I October
PHYSICS LETfERS
phase on silia
surfaas Assignment
Adsorbed
(cm’
)
1976
Gel
A-%
802 1027 1268 1443
1.0 0.45 0.45 0.50
802 1026 1267 1444
0.95 0.77 1.14
depol. dcpoL depot.
2851 2923 2937
0.91 1.0 0.92
2852 2922 2937
0.74 1.0 0.91
potPaL pot
609 990 1178 1587 1606 2947 3060
0.02 1.00 0.08 0.08 0.06 0.12 1.48
607 991 1176 1586 1607 2947 3061
0.11 1 .oo 0.21 0.16 0.13 0.14 2.53
depot POL depot depoL depol. pot POL
262 366 670
0.30 0.68 1.00
263 366 669
1.28 1.04 1.00
216 313 453 456 460 759 789
0.37 0.37
218 313
0.68 0.71
dcpol. dcpoL
1.00
460
1.00
pot.
0.34 0.38
760 790
0.38) 0.61
350 850 1270 1575
0.60 0.40 0.93 1.00
351 850 1273 1578
1.87 0.49 1.10 l.rJO
238 350 448 512
0.45 0.13 1.00 0.10
239 350 450 513
0.62 0.27 1.00 0.06
19s 687 874 939
0.87 1.00 0.24 0.20
198 689 875 941
1.68 1.00 0.24 0.38
2235 2731 2913 2933 2983
0.36 0.10 0.70 1.00 0.86
2239 2731 2913 2935 2983
0.50 0.12 0.77 1.00 t.38
a) Two reference bands used in different b, Assignments according to ref. [ 121. c) Assignments according to ref. [ 131.
spectral
regions due to necessary
km-’
)
Iret
1.0
changes in reoording
POL
u3tfihpr
vsbg)
+u4(F2)
depot
POL
u4(ag)
po[-
u3fq
pof-
dag)
pal.
u3(9
POL
ua(b$
dcpol. potdepot
*fag) “afbzg) ?26@
&&I 1
depoL pot
u7bt)
pot
v22W
depol.
~(81)
pof-
h6
WI-
vt q4*
pot,
~1SW
‘depoL
conditions
39
CHEMICAL PHYSICS LETTERS
Volume 43, number 1
to different degrees. For benzene e.g. the TL mode at 3062 cm-’ IS * particularly influenced and this observation is also made for adsorbed benzene (see table I). We therefore believe that the intluencc of surface electric fields must be at least one cf the more important factors which lead to the enhancement of Raman band intensities on adsorption. As shown by Aussenegg and co-workers [ 111 the intensity enhancement by the electric field is accompanied by an increase of the depolarization ratio. This value can unfortunately not be determined for molecules adsorbed on polydisperse adsorbcnts, since the superimposed Rayleigh scattering leads to a compiete depolarization of all Raman lines. It is believed that the intensity changes are related to the strongly directional nature of the adsorption forces and the resulting orientation of the adsorbed molecules relative to the surface. Unfortunately, the theories of Raman intensities of condensed
phases are still not well developed
and the
influence of electric fields is not understood theoretically. A clear relation between experimentally observed intensity changes and orientations of adsorbed molecules can thus not be worked out ar present. The authors wish to thank Dr. W. Kiefer and Professor Dr. H.W. Schriitter
for valuable
discussions.
Financial
support by the Deutsche Forschungsgemeinschaft gratefully acknowledged.
40
is
1 October 1976
References R.P. Cooney. G. Curthoys and Nguyen The Tam. Advan. Catalysis 24 (1975) 293. T.A. Egerton and A.H. Hardin, Catal. Rev. Sci Eng. 11 (1974) 1. E. Bucchler and J. Turkevich, J. Phys Chcm. 76 (1972) 2325. T.A. Egcrton, A.H. Hardin. Y. Kozirovski and N. Shcp pard, J. Catalysis 32 (1974) 343. P.J. Hcndra and El. Loader. Trans Faraday Sot 67
(1971) 828. Nguyen The Tam, R.P. Cooncy and G. Curthoys, J. Calloid Interface ScL 51 (1975) 340. H. Kn&ingcr and It. Krictcnbrink, J. Chcm. Sot. Faraday 171 (1975) 2421. D.A. Bahnick and W.B. Person, J. Chcm. Phyr 48 (1968)
1251. P.J. He&a.
J.D.M. Turner, E.J. Loader and M. Stacey, J. Phyr Chem 78 (1974) 300. [IO] H. Knozinger. in: Hydrogen bonding - recent develop mcnts in theory and cxperimcnts, cdr P. Schuster. G. Zundcl and C. Sandorfy (North-Holland, Amsterdam, 1976). [ 111 F. Aussencgg, ht. Lippitsch, R. Molter and I. Wagner, Phys. Letters SOA (1974) 233. 1121 G. Hcrzberg, Infrared and Raman spectra of polyatomic molecules (Van Nostrand, Princeton, 1954). [ 13 j G.A. Crowder, J. Phys. Chcm. 75 (1971) 2806.
CHEMICAL PHYSICS LETTERS
1 Octobn
1976
LASER RAMAN SPECTRA OF MOLECULES ADSORBED ON SILICA SURFACES H. JEZIOROWSKJ and H. KNOZINCER lnstitut
ftir Physikolitche
Chemie. ilnivenit&
* Mixhen.
8 Munich 2, West Germany
Received 25 June 1976 The laser Raman spectra of chloroform, carbon tetnchloride, trans-1,2dichloroethylene, tctrachlorocthylene. tetramethytethylene. wt.-butyl cyanide, benzene and cyclohcxane adsorbed on silica surfaces have been remrded. The Raman shifts of the adsorbed molecules do not significantly dcviatc from those of the liquids and the Raman selection rules arc not viotted by the adsorption The relative intensities of the Raman bands of adsorbed species arc distinct from those of the liquids. Rusons for these intensity changes on adsorption arc discusscd.?hc surfaa electric field is found to be an important factor. .
laser Raman spectra of a variety of unsaturated molecules, which were adsorbed on the surfaces of silica or porous glass have been reported previously [ I,2 1, whereas Raman spectroscopy seems not to have been applied to studies of more weakly adsorbed molecules. such as e.g. saturated hydrocarbons. The Raman shifts of the molecules adsorbed on silicas usually very closely resembled that of the corresponding liquids, however, significant deviations of relative band intensities of adsorbed benzene [3,4], carbon tetrc chloride (5) and thiophene (61 from those in the liquid phase were observed. Analogous results were obtained by KnSzinger and Krietenbrink [7] with ferr.-butyl cyanide weakly adsorbed on v-AJ~ CJ3. This phenomenon was explained as being due to the specific H-bonding interactions between the surface hydroxyl groups and n-electron systems [4,6] and/or by the effect of the refractive index in connection with the directional nature of the adsorption interaction [6]. Bahnick and Person [8] have shown, that weak charge transfer interactions influence the relative intensrties of CCl, in solution. Such interactions may also occur on silica surfaces. However, the intensity increases reported by Bahtick and Person [8] were only by a factor of 1.6 for the strongest electron donor molecules, whereas the relative intensities of certain Raman lines of adsorbed molecules changed by factors as high as 2 to 4 [7] (see also table 1). We have therefore measured the relative intensities of various molecules adsorbed on silica, including cycloThe
and halogenated
hexane as an example of a saturated hydrocarbon w’tich cannot undergo specific interactions. tins-1,2dichIoroethylene, tetrachloroethylene. tetramethylethylene and benzene - molecules which posses a center of symmetry - were also included so that a possible violation of RIman selection rules due to the adsorption interaction must be detectable by the appearance of new Raman bands. The spectra were recorded on a Cary 82 spectrometer using the 90” optics. The greaseI_s sample tubes were similar to those described by Hendra and co-workers [9 J. The argon ion laser line at 5 14.5 nm was used as the exciting line, the laser output being set at 200 mW. ‘this laser power did not cause any thermal desoX+ion; the Raman line intensities increased linearly on laser excitation with output power up to 600 mW and started dccreasing only at higher excitation power. Standard recording conditions are given in the legend to fig. I. The silica used as adsorbent was AEROSCL 200 from Degussa with an N, BET surface area of 170 m2/g. The oxide powder was slightly pressed into the samole tube and treated in an 0, atmosphere at 500°C bef&e evacuation. This procedure very effectively eliminated serious “fluorescence** problems. The adsorptives were all p.A. grade and dried over Linde molecuhrr sieve 3A before use. They were adsorbed on the silica under their saturation vapour pressure at room temperature. The Raman spectra of liquid CC$ and of CC+ adsorbed on silica are compared in fig. 1. The positions of the bands are practically identical in both phases and the 37
Volume 43, number 1
CHEMICAL PHYSICS LETTERS
b
Fig. 1. Raman spectra of liquid C-(a) and Cf& adsorbed on silica (b). (a) Spectral slit width: 0.5 cm-‘, scan speed 1 cm-‘/s; pen period: 1 s; sensitivity 5000 counts/s; excitation: 514.5 nm. 600 mW; (b) spectral slit width: 0.75 cm-‘. scan speed: 0.1 cm?/s, pen period: 20 s, sensitivity: 200 counts/s, excitation: 514.5 nm. 200 mW.
same conclusion holds for nearly all the observed Raman bands of the adsorption systems studied. As seen from the data summarized in table 1, the maximum difference in band position is 3 cm” in the case of nuns-l,?_dichloroethylene. Another important observation is, that no IR active bands of the free molecules with center of symmetry appear in the Raman spectra of the adsorbed molecules. In these cases the principle of mutual exclusion which holds for the free molecules, might be expected to become invalid for the adsorbed species. A perturbation of the molecular symmetry must indeed be induced by the interaction with the solid surface, since IR forbidden transitions are activated and arc observed in the IR spec tra of the a&orbed molecules. E.g. strong bands are detected at 1712 and 1656 cm” on adsorption of tetramethylethylene and frrrn=-1,2dichloroethylene, respec.38
1 Octoba
1976
tively, which must be-assigned as the stretching vibration of the C=C double bond. It must therefore be concluded that neither specific nor unspecific interactions of the adsorbed molecules with the surface cause sufficiently severe distortions of the molecular symmetry as to produce significant Raman band displacements or to activate Raman inactive transitions. These observations and conclusions are in agreement, with those reported by Buechler and Turkevich [3], Egerton et al. (4) and Cooncy and co-workers [6). Fig. 1 also demonstrates that the relative intensities of the CCl, Raman bands are significantly altered in the adsorbed state as compared to the liquid. The intensity data for the other systems are summarized in table 1. These data clearly show that the various normal modes of a given molecule are affected differently by the adsorption. The most remarkable effects are observed for chloroform, for which the relative intensity of the r6 mode at 262 cm” is enhanced by a factor of 4.3. But even in the case of cyclohexane - a nonpolar molecule which should riot be able to undergo any kind of specific adsorption interaction - the relative intensity of the band at 1442 cm-t is enhanced by a factor of 2.2. Unsaturated and halogen containing hydrocarbons cause characteristic perturbations of surface hydroxyl groups when they are adsorbed on silica surfaces [lo] and these perturbations indicate specific adsorption interactions of the H-bond type. H-bonding may lead to intensity changes as suggested by Cooney and co-workers in the case of thiophene adsorption [6]. The formation of weak charge transfer complexes may be responsible for the observed phenomena with chlorine containing compounds, as proposed for CCl, in solvents of varying electron donor strength by Bahnick and Person [8]. These effects, however, cannot account for the intensity changes of theRaman bands of cyclohexane. Even in a structured liquid, the molecules “see” a nearly isotropic environment, whereas an adsorbed molecule on a solid surface is located in an extremely anisotropic environment and must be distorted and polarized by the strong surface electric fields which are known to reach values of the order of magnitude of lo6 V/cm. Aussenegg et al. [l 1) have recently reported intensity measurements of Raman lines of liquids in electric fields (0 to S x 10s V/cm). They observe significant intensity enhancements of certain Raman bands with increasing field strength and different normal modes are affected
Volume 43, nurnbcr
1
Tabie 1 Relative Raman intensities
CHEMlCAL
in liquids and in the adsorbed Liquid
Compound
AIR
cydohexane
benzene
a)
b,
chloroform
b,
carbon tetrachloridc
b)
@rms-1,2_dichlorocthylcnc
tetrahloro
ethyiene
b,
ferf.-butyl cyanide W)
b,
I October
PHYSICS LETfERS
phase on silia
surfaas Assignment
Adsorbed
(cm’
)
1976
Gel
A-%
802 1027 1268 1443
1.0 0.45 0.45 0.50
802 1026 1267 1444
0.95 0.77 1.14
depol. dcpoL depot.
2851 2923 2937
0.91 1.0 0.92
2852 2922 2937
0.74 1.0 0.91
potPaL pot
609 990 1178 1587 1606 2947 3060
0.02 1.00 0.08 0.08 0.06 0.12 1.48
607 991 1176 1586 1607 2947 3061
0.11 1 .oo 0.21 0.16 0.13 0.14 2.53
depot POL depot depoL depol. pot POL
262 366 670
0.30 0.68 1.00
263 366 669
1.28 1.04 1.00
216 313 453 456 460 759 789
0.37 0.37
218 313
0.68 0.71
dcpol. dcpoL
1.00
460
1.00
pot.
0.34 0.38
760 790
0.38) 0.61
350 850 1270 1575
0.60 0.40 0.93 1.00
351 850 1273 1578
1.87 0.49 1.10 l.rJO
238 350 448 512
0.45 0.13 1.00 0.10
239 350 450 513
0.62 0.27 1.00 0.06
19s 687 874 939
0.87 1.00 0.24 0.20
198 689 875 941
1.68 1.00 0.24 0.38
2235 2731 2913 2933 2983
0.36 0.10 0.70 1.00 0.86
2239 2731 2913 2935 2983
0.50 0.12 0.77 1.00 t.38
a) Two reference bands used in different b, Assignments according to ref. [ 121. c) Assignments according to ref. [ 131.
spectral
regions due to necessary
km-’
)
Iret
1.0
changes in reoording
POL
u3tfihpr
vsbg)
+u4(F2)
depot
POL
u4(ag)
po[-
u3fq
pof-
dag)
pal.
u3(9
POL
ua(b$
dcpol. potdepot
*fag) “afbzg) ?26@
&&I 1
depoL pot
u7bt)
pot
v22W
depol.
~(81)
pof-
h6
WI-
vt q4*
pot,
~1SW
‘depoL
conditions
39
CHEMICAL PHYSICS LETTERS
Volume 43, number 1
to different degrees. For benzene e.g. the TL mode at 3062 cm-’ IS * particularly influenced and this observation is also made for adsorbed benzene (see table I). We therefore believe that the intluencc of surface electric fields must be at least one cf the more important factors which lead to the enhancement of Raman band intensities on adsorption. As shown by Aussenegg and co-workers [ 111 the intensity enhancement by the electric field is accompanied by an increase of the depolarization ratio. This value can unfortunately not be determined for molecules adsorbed on polydisperse adsorbcnts, since the superimposed Rayleigh scattering leads to a compiete depolarization of all Raman lines. It is believed that the intensity changes are related to the strongly directional nature of the adsorption forces and the resulting orientation of the adsorbed molecules relative to the surface. Unfortunately, the theories of Raman intensities of condensed
phases are still not well developed
and the
influence of electric fields is not understood theoretically. A clear relation between experimentally observed intensity changes and orientations of adsorbed molecules can thus not be worked out ar present. The authors wish to thank Dr. W. Kiefer and Professor Dr. H.W. Schriitter
for valuable
discussions.
Financial
support by the Deutsche Forschungsgemeinschaft gratefully acknowledged.
40
is
1 October 1976
References R.P. Cooney. G. Curthoys and Nguyen The Tam. Advan. Catalysis 24 (1975) 293. T.A. Egerton and A.H. Hardin, Catal. Rev. Sci Eng. 11 (1974) 1. E. Bucchler and J. Turkevich, J. Phys Chcm. 76 (1972) 2325. T.A. Egcrton, A.H. Hardin. Y. Kozirovski and N. Shcp pard, J. Catalysis 32 (1974) 343. P.J. Hcndra and El. Loader. Trans Faraday Sot 67
(1971) 828. Nguyen The Tam, R.P. Cooncy and G. Curthoys, J. Calloid Interface ScL 51 (1975) 340. H. Kn&ingcr and It. Krictcnbrink, J. Chcm. Sot. Faraday 171 (1975) 2421. D.A. Bahnick and W.B. Person, J. Chcm. Phyr 48 (1968)
1251. P.J. He&a.
J.D.M. Turner, E.J. Loader and M. Stacey, J. Phyr Chem 78 (1974) 300. [IO] H. Knozinger. in: Hydrogen bonding - recent develop mcnts in theory and cxperimcnts, cdr P. Schuster. G. Zundcl and C. Sandorfy (North-Holland, Amsterdam, 1976). [ 111 F. Aussencgg, ht. Lippitsch, R. Molter and I. Wagner, Phys. Letters SOA (1974) 233. 1121 G. Hcrzberg, Infrared and Raman spectra of polyatomic molecules (Van Nostrand, Princeton, 1954). [ 13 j G.A. Crowder, J. Phys. Chcm. 75 (1971) 2806.