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Fourier transform infrared studies of ethylene and acetylene adsorbed on a silica-supported platinum catalyst
Vibrational Spectroscopy,
2 (1991) 29-32
Elsevier science Publishers B.V., Amsterdam
Fourier transform infrared studies of ethylene and acetylene adsorbed on a silica-supported platinum catalyst Tibor Szilagyi Institute of Isotopes, Hungarian Academy of Sciences, P. 0. B. 77, H-l 525 Budapest (Hungary)
(Received 9th April 1990)
Abstract Infrared spectra of adsorbed ethylene and acetylene and the effect of hydrogen on the surface species are discussed. In addition to the known ethylidyne form, at high coverages ethylene is also present as a vinyl group on the surface. Acetylene is probably dimerized and gradually hydrogenated, but only partially desorbed as a saturated product. Keywords: Infrared spectrometry; Acetylene; Ethylene; Platinum/silica catalyst
Infrared (IR) spectrometry has contributed substantially to the understandi ng of the complex nature of surface and catalytic processes. Although modem surface science methods (e.g., electron spectroscopy or diffraction) offer valuable information about surface species formed on socalled “model surfaces” (single crystals, etc.) in ultrahigh vacuum, IR spectrometry is indispensable when disperse catalysts under considerable adsorbate pressures are to be investigated. The IR spectra generally show that several surface compounds coexist on the metal surface and, in spite of the numerous efforts made to explain them, contradictions still persist in the literature. The situation is even more complicated as the structure of the metal surface in supported disperse catalysts is always poorly characterized so the adsorption site cannot be determined unequivocally. In this paper, some of these problems are discussed with the help of some preliminary results for a series of measurements aimed at the study of hydrocarbons adsorbed on Pt/SiO, catalysts. I
0924-2031/91/$03.50
0 1991 - Elsevier Science Publishers B.V.
The adsorption of ethylene platinum has been studied on different single crystal surfaces [l-8] and on supported catalysts by IR [9-131 and NMR spectrometry [14-161. It is generally agreed that at low temperatures ethylene is adsorbed in a d&a-bonded form, but opinions differ on the room-temperature surface structure, ethylidene [6,7], ethylidyne [1,2,11,13,15], a-vinyl [4] and ITethylene [10,12] forms having been suggested. Room-temperature adsorption of acetylene has been explained by assuming the presence of vinylidene [4] or ethylidene [7] species on a Pt(ll1) surface.
EXPERIMENTAL
A Pt/SiO, catalyst with a high metal loading was prepared by impregnating Cab-O-Sil-HS5 silica (BET surface area 300 m* g-l) with a solution of H,PtC1,. The properties of the catalyst were as follows: metal content, 16.3% (w/w); CO chemisorbed at 300 IS, 152 pg g-i catalyst; disper-
30
sion, 0.21; average particle size, 6.5 nm; and specific surface area, 7.0 m* g-‘. The dried catalyst was pressed into a thin pellet (10 mg cm-*), calcined in air at 720 K and reduced in a flow (8 1 h-‘) of pure hydrogen for 2 h at 620 K in the adsorption cell. The cell and vacuum system have been described elsewhere [17]. Both were constructed of stainless steel and were essentially suitable for UHV standards. However, the present conditions (use of a relatively high pressure of gases, lack of bakeout) allowed a dynamic vacuum of only 10B5 Pa to be reached. Dispersion (as the ratio of surface metal atoms to total Pt atoms, Pt,/Pt t) was calculated from CO chemisorption data measured gravimetrically in a Sartorius microbalance on catalyst samples pretreated under the same conditions. The average particle diameter was calculated assuming a spherical particle shape and using a value of the surface atomic density of 1.5 X 10” atoms cm-* which takes into account the known reconstruction of certain Pt crystal faces [18] [if the value of 1.24 X 1015 atoms cm-*, i.e., the average of the atomic densities of the unreconstructed low index faces (ill), (llO), (lOO), is used, a smaller particle size results (5.4 nm)]. The average crystallite size was also measured by x-ray diffraction and gave a value of 7.2 nm. For this study these small differences are not significant. Such a relatively large particle size means that the so-called “metal surface selection rule” should be taken into account [19]. As a consequence, only those vibrations having a dynamic dipole moment perpendicular to surface can be observed in the spectra. After completing the reduction, the samples were either allowed to cool in a hydrogen atmosphere and evacuated at room temperature or the hydrogen flow was stopped and the cell was evacuated at the reduction temperature and subsequently cooled under vacuum. In the former instance the samples are believed to be covered with a monolayer of strongly bonded hydrogen (i.e., non-evacuable at 300 K), so they are “hydrogencovered” [20]. Samples treated in a manner similar to the latter are often referred to as “hydrogenfree”. Nevertheless, in this work samples cooled under vacuum were found to contain a significant
T. SZItiGYI
amount of strongly bound hydrogen. Although the cell was not constructed for accurate volumetric measurements, a rough estimation of the amount adsorbed was possible in certain instances (i.e., when introduction of a known amount of gas to the sample resulted in a relatively large pressure drop in the cell as a result of strong adsorption), Thus it was found that samples evacuated at 620 K were still covered with ca. 0.3 monolayer of strongly bound hydrogen [21]. Adsorption was applied at room temperature on both hydrogen-covered and “hydrogen-free” samples. Gases were introduced into the cell at several different pressures between 5 and 40 Pa. After a 2-min contact time the cell was evacuated and spectra were recorded. The spectra were virtually identical in all instances (except that the intensity of bands of the gas phase changed corresponding to the pressure change, of course), indicating that the adsorption is complete in the investigated pressure region in accordance with other observations [l-8], i.e., the surface is saturated with the ethylidyne form already at much lower pressures (lo-’ Pa) with a coverage of 0.25. However, as discussed below, extra spectral features can only be explained by assuming other coexisting surface species, i.e., in this instance either the coverage is higher or the layer is reconstructed giving rise to new surface compounds. Infrared spectra were measured on a Digilab FTS-20C Fourier transform spectrophotometer at a nominal resolution of 4 cm- ‘. First the spectrum of the “pure” sample was taken, stored on disc and used as the background later.
RESULTS
AND DISCUSSION
Spectra of adsorbed ethylene are shown on Figs. 1 and 2a. In the gas phase a considerable amount of ethane appeared on adsorption on both hydrogen-free and -covered samples, indicating that ethylene not only is hydrogenated by residual surface hydrogen but also adsorbs dissociatively and self-hydrogenation occurs. In the spectrum of the chemisorbed phase two strong, sharp bands at 2890 and 1345 cm-’ can be assigned to the stable ethylidyne structure observed in electron energy
31
FT-IROFETHYLENEANDACETYLENE
I
I
1
3200
m
1
I
2800
3000
ci
Fig. 1. Spectra of adsorbed ethylene: (b) after addition of 28 Pa hydrogen.
0a chemisorbed
loss spectra and deduced from low-energy electron diffraction studies [1,2,22]. The band at 2805 cm-’ is the overtone of the asymmetric deformation of the ethylidyne methyl group forbidden by the metal surface selection rule. Bands at 3020, 2995, 1505 and 1420 cm-l indicate the presence of another, unsaturated form of chemisorbed ethylene on the Pt surface. This unsaturated form of adsorbed ethylene was reported earlier as a Ir-complex [ll]. However, the relatively high intensity of the 1505 cm-’ band (C=C stretch) shows that this compound cannot be parallel to the surface, because if it were then the C=C band should be inactive in the sense of
I 0.005 I
k
,
1800
I
1
I
lSO0
I
4
Crii’
Fig. 2. Low-frequency region of (a) chemisorbed ethylene and (b) chemisorbed acetylene.
the metal surface selection rule. Moreover, the surface is already covered by ethylidyne and, owing to a lack of space, the formation of the bulky Ir-complex is unlikely. Hence these bands should be assigned to another surface compound, probably a o-bonded vinyl group that appears only at high coverages after the hydrogen has been removed and the surface is saturated by ethylidyne. On some of the spectra a weak band is discemible at 2925 cm-l, which can be tentatively assigned to the asymmetric stretching of a CH, group. If this is so, a third kind of adsorbed ethylene should also be present, which is probably the di-a-bonded Pt-CH,-CH,-Pt reported previously [9,23]. This complex should have staggered conformation (II) because in the eclipsed form (I) the dipole moment change is parallel to the surface and hence the discussed vibration is inactive. As a consequence of the staggered conformation, the Pt-Pt and C-C bonds are not parallel (III) [24].
H
H
Yk H
H
/
H
Pt
Pt
Pt
t k
C
H
H
Pt
C
Pt
(1)
(11)
(III)
Addition of hydrogen to the sample and subsequent evacuation resulted in the disappearance of all the bands of the chemisorbed species and the formation of gas-phase ethane, indicating that all forms of adsorbed ethylene are easily hydrogenated. The adsorption of acetylene resulted in complicated spectra as shown in Figs. 2b and 3. On introducing the gas into the cell, the pressure drops to about the half of the initial value and the gas phase contains no acetylene but consists of ethane only. Taking into account the volume of the cell (ca. 1 l), this means that a large amount of acetylene is dehydrogenated (and self-hydrogenated) and a carbon deposit is also formed on the surface. It is seen that these spectra cannot be explained by assuming the simple structures found
32
T. SZItiGYI
Conclusions Infrared spectra of ethylene and acetylene adsorbed at pressures higher than 5 Pa suggest that the surface layer consists of several different species, in contrast to the simple ordered structures observed under high vacuum. After adsorption ethylene is also present as vinyl groups at high coverages in addition to ethylidyne (known from previous experiments). Acetylene is concluded to be dimerized and gradually hydrogenated. It desorbs, only in part, however, as a saturated product.
a
b
3000
2800
2800
cm
REFERENCES
Fig. 3. Spectra of adsorbed acetylene: (a) together with the gas phase after introduction of acetylene; (b) gas phase evacuated; (c) (a-b) difference spectrum; (d) after addition of 28 Pa hydrogen; (e) same as (d) after evacuation; (f) (d-e) difference spectrum.
on single-crystal surfaces [4,7]. Bands above 3000 cm-’ and at 1700,142O cm-l show that acetylene chemisorbs initially in unsaturated form. This is probably vinylidene and the high frequency of the C=C bond (1700 cm-‘) is caused by the Pt --) C=C electron transfer. Another possibility is that the vinylidene is dimerized and the high-frequency (in-phase) component of the interacting C=C vibrations is observed in the spectra (Fig. 4). This dimer is gradually hydrogenated, as is indicated by the strong methyl bands at 2970, 2880 and 1460 cm-l. Addition of hydrogen to samples covered with chemisorbed acetylene increased the intensity of the CH, band but evacuation restored the original spectrum Such cycles can be performed several times without significant degradation of the spectra (only a small amount of ethane is formed), indicating that strongly bound hydrogen-deficient species (carbon deposit) also exist on the surface, and that it can reversibly take up hydrogen but it cannot be saturated and desorbed completely. 3 c II Pt
N
v
c-c I Pt
\ Pt
""\ c-c
C"3 /
I\ I\ Pt Pt Pt Pt
3 3
7
C"3
/ CH-CH / Pt
\
Pt
Fig. 4. Possible surface reactions of chemisorbed acetylene.
1 L.L. Kesmodel, L.H. Dubois and G.A. Somorjai, J. Chem. Phys., 70 (1979) 2180. 2 P.C. Stair and G.A. Somorjai, J. Chem. Phys., 66 (1977) 573. 3 T.E. Fischer and S.R. Kelemen, J. Vat. Sci. Technol., 15 (1978) 607. 4 J.E. Demuth, Surf. Sci., 80 (1979) 367. 5 M.R. Albert, L.G. Sneddon, W. Eberhardt, F. Greuter, T. Gustaffson and E.W. Plummer, Surf. Sci., 120 (1982) 19. 6 H. Ibach and S. LehwaId, J. Vat. Sci. Technol., 15 (1978) 407. 7 H. Ibach, H. Hopster and B. Sexton, Appl. Surf. Sci., 1 (1977) 1. 8 E. Yagasaki and RI. Masel, Surf. Sci. 226 (1990) 51. 9 B.A. Morrow and N. Sheppard, Proc. R. Sot. London, Ser. A, 311 (1969) 391. 10 J.D. Prentice, A. Lesiunas and N. Sheppard, J. Chem. Sot., Chem. Commun., 76 (1976). 11 N. Sheppard and C. de la Cruz, React. Kinet. Catal. Lett., 35 (1987) 21. 12 Y. Soma, J. CataI., 59 (1979) 239. 13 T.P. Beebe and J.T. Yates, Jr., J. Phys. Chem., 91 (1987) 254. 14 T. Shibanuma and T. Matsui, Surf. Sci., 154 (1985) L215. 15 P.K. Wang, C.P. SIichter and J.H. Sinfelt, J. Phys. Chem., 89 (1985) 3060. 16 I.D. Gay, J. Catal., 108 (1987) 15. 17 T. Szilagyi, PhD Thesis, Institute of Isotopes of the Hungarian Academy of Sciences, Budapest, 1986. 18 R.J. Koestner, M.A. Van Hove and G.A. Somorjai, J. Phys. Chem., 87 (1983) 203. 19 H.A. Pearce and N. Sheppard, Surf. Sci., 59 (1976) 205. 20 J.D. Clewley, J.F. Lynch and T.B. Flanagan, J. Catal., 36 (1975) 291. 21 T. Szil&yi, J. Catal., 121 (1990) 223. 22 M. Procop and J. Viilter, Surf. Sci., 33 (1972) 69. 23 B.A. Morrow and N. Sheppard, J. Phys. Chem., 70 (1966) 2406. 24 N. Sheppard and J. Erkelens, Appl. Spectrosc., 38 (1984) 471.
2 (1991) 29-32
Elsevier science Publishers B.V., Amsterdam
Fourier transform infrared studies of ethylene and acetylene adsorbed on a silica-supported platinum catalyst Tibor Szilagyi Institute of Isotopes, Hungarian Academy of Sciences, P. 0. B. 77, H-l 525 Budapest (Hungary)
(Received 9th April 1990)
Abstract Infrared spectra of adsorbed ethylene and acetylene and the effect of hydrogen on the surface species are discussed. In addition to the known ethylidyne form, at high coverages ethylene is also present as a vinyl group on the surface. Acetylene is probably dimerized and gradually hydrogenated, but only partially desorbed as a saturated product. Keywords: Infrared spectrometry; Acetylene; Ethylene; Platinum/silica catalyst
Infrared (IR) spectrometry has contributed substantially to the understandi ng of the complex nature of surface and catalytic processes. Although modem surface science methods (e.g., electron spectroscopy or diffraction) offer valuable information about surface species formed on socalled “model surfaces” (single crystals, etc.) in ultrahigh vacuum, IR spectrometry is indispensable when disperse catalysts under considerable adsorbate pressures are to be investigated. The IR spectra generally show that several surface compounds coexist on the metal surface and, in spite of the numerous efforts made to explain them, contradictions still persist in the literature. The situation is even more complicated as the structure of the metal surface in supported disperse catalysts is always poorly characterized so the adsorption site cannot be determined unequivocally. In this paper, some of these problems are discussed with the help of some preliminary results for a series of measurements aimed at the study of hydrocarbons adsorbed on Pt/SiO, catalysts. I
0924-2031/91/$03.50
0 1991 - Elsevier Science Publishers B.V.
The adsorption of ethylene platinum has been studied on different single crystal surfaces [l-8] and on supported catalysts by IR [9-131 and NMR spectrometry [14-161. It is generally agreed that at low temperatures ethylene is adsorbed in a d&a-bonded form, but opinions differ on the room-temperature surface structure, ethylidene [6,7], ethylidyne [1,2,11,13,15], a-vinyl [4] and ITethylene [10,12] forms having been suggested. Room-temperature adsorption of acetylene has been explained by assuming the presence of vinylidene [4] or ethylidene [7] species on a Pt(ll1) surface.
EXPERIMENTAL
A Pt/SiO, catalyst with a high metal loading was prepared by impregnating Cab-O-Sil-HS5 silica (BET surface area 300 m* g-l) with a solution of H,PtC1,. The properties of the catalyst were as follows: metal content, 16.3% (w/w); CO chemisorbed at 300 IS, 152 pg g-i catalyst; disper-
30
sion, 0.21; average particle size, 6.5 nm; and specific surface area, 7.0 m* g-‘. The dried catalyst was pressed into a thin pellet (10 mg cm-*), calcined in air at 720 K and reduced in a flow (8 1 h-‘) of pure hydrogen for 2 h at 620 K in the adsorption cell. The cell and vacuum system have been described elsewhere [17]. Both were constructed of stainless steel and were essentially suitable for UHV standards. However, the present conditions (use of a relatively high pressure of gases, lack of bakeout) allowed a dynamic vacuum of only 10B5 Pa to be reached. Dispersion (as the ratio of surface metal atoms to total Pt atoms, Pt,/Pt t) was calculated from CO chemisorption data measured gravimetrically in a Sartorius microbalance on catalyst samples pretreated under the same conditions. The average particle diameter was calculated assuming a spherical particle shape and using a value of the surface atomic density of 1.5 X 10” atoms cm-* which takes into account the known reconstruction of certain Pt crystal faces [18] [if the value of 1.24 X 1015 atoms cm-*, i.e., the average of the atomic densities of the unreconstructed low index faces (ill), (llO), (lOO), is used, a smaller particle size results (5.4 nm)]. The average crystallite size was also measured by x-ray diffraction and gave a value of 7.2 nm. For this study these small differences are not significant. Such a relatively large particle size means that the so-called “metal surface selection rule” should be taken into account [19]. As a consequence, only those vibrations having a dynamic dipole moment perpendicular to surface can be observed in the spectra. After completing the reduction, the samples were either allowed to cool in a hydrogen atmosphere and evacuated at room temperature or the hydrogen flow was stopped and the cell was evacuated at the reduction temperature and subsequently cooled under vacuum. In the former instance the samples are believed to be covered with a monolayer of strongly bonded hydrogen (i.e., non-evacuable at 300 K), so they are “hydrogencovered” [20]. Samples treated in a manner similar to the latter are often referred to as “hydrogenfree”. Nevertheless, in this work samples cooled under vacuum were found to contain a significant
T. SZItiGYI
amount of strongly bound hydrogen. Although the cell was not constructed for accurate volumetric measurements, a rough estimation of the amount adsorbed was possible in certain instances (i.e., when introduction of a known amount of gas to the sample resulted in a relatively large pressure drop in the cell as a result of strong adsorption), Thus it was found that samples evacuated at 620 K were still covered with ca. 0.3 monolayer of strongly bound hydrogen [21]. Adsorption was applied at room temperature on both hydrogen-covered and “hydrogen-free” samples. Gases were introduced into the cell at several different pressures between 5 and 40 Pa. After a 2-min contact time the cell was evacuated and spectra were recorded. The spectra were virtually identical in all instances (except that the intensity of bands of the gas phase changed corresponding to the pressure change, of course), indicating that the adsorption is complete in the investigated pressure region in accordance with other observations [l-8], i.e., the surface is saturated with the ethylidyne form already at much lower pressures (lo-’ Pa) with a coverage of 0.25. However, as discussed below, extra spectral features can only be explained by assuming other coexisting surface species, i.e., in this instance either the coverage is higher or the layer is reconstructed giving rise to new surface compounds. Infrared spectra were measured on a Digilab FTS-20C Fourier transform spectrophotometer at a nominal resolution of 4 cm- ‘. First the spectrum of the “pure” sample was taken, stored on disc and used as the background later.
RESULTS
AND DISCUSSION
Spectra of adsorbed ethylene are shown on Figs. 1 and 2a. In the gas phase a considerable amount of ethane appeared on adsorption on both hydrogen-free and -covered samples, indicating that ethylene not only is hydrogenated by residual surface hydrogen but also adsorbs dissociatively and self-hydrogenation occurs. In the spectrum of the chemisorbed phase two strong, sharp bands at 2890 and 1345 cm-’ can be assigned to the stable ethylidyne structure observed in electron energy
31
FT-IROFETHYLENEANDACETYLENE
I
I
1
3200
m
1
I
2800
3000
ci
Fig. 1. Spectra of adsorbed ethylene: (b) after addition of 28 Pa hydrogen.
0a chemisorbed
loss spectra and deduced from low-energy electron diffraction studies [1,2,22]. The band at 2805 cm-’ is the overtone of the asymmetric deformation of the ethylidyne methyl group forbidden by the metal surface selection rule. Bands at 3020, 2995, 1505 and 1420 cm-l indicate the presence of another, unsaturated form of chemisorbed ethylene on the Pt surface. This unsaturated form of adsorbed ethylene was reported earlier as a Ir-complex [ll]. However, the relatively high intensity of the 1505 cm-’ band (C=C stretch) shows that this compound cannot be parallel to the surface, because if it were then the C=C band should be inactive in the sense of
I 0.005 I
k
,
1800
I
1
I
lSO0
I
4
Crii’
Fig. 2. Low-frequency region of (a) chemisorbed ethylene and (b) chemisorbed acetylene.
the metal surface selection rule. Moreover, the surface is already covered by ethylidyne and, owing to a lack of space, the formation of the bulky Ir-complex is unlikely. Hence these bands should be assigned to another surface compound, probably a o-bonded vinyl group that appears only at high coverages after the hydrogen has been removed and the surface is saturated by ethylidyne. On some of the spectra a weak band is discemible at 2925 cm-l, which can be tentatively assigned to the asymmetric stretching of a CH, group. If this is so, a third kind of adsorbed ethylene should also be present, which is probably the di-a-bonded Pt-CH,-CH,-Pt reported previously [9,23]. This complex should have staggered conformation (II) because in the eclipsed form (I) the dipole moment change is parallel to the surface and hence the discussed vibration is inactive. As a consequence of the staggered conformation, the Pt-Pt and C-C bonds are not parallel (III) [24].
H
H
Yk H
H
/
H
Pt
Pt
Pt
t k
C
H
H
Pt
C
Pt
(1)
(11)
(III)
Addition of hydrogen to the sample and subsequent evacuation resulted in the disappearance of all the bands of the chemisorbed species and the formation of gas-phase ethane, indicating that all forms of adsorbed ethylene are easily hydrogenated. The adsorption of acetylene resulted in complicated spectra as shown in Figs. 2b and 3. On introducing the gas into the cell, the pressure drops to about the half of the initial value and the gas phase contains no acetylene but consists of ethane only. Taking into account the volume of the cell (ca. 1 l), this means that a large amount of acetylene is dehydrogenated (and self-hydrogenated) and a carbon deposit is also formed on the surface. It is seen that these spectra cannot be explained by assuming the simple structures found
32
T. SZItiGYI
Conclusions Infrared spectra of ethylene and acetylene adsorbed at pressures higher than 5 Pa suggest that the surface layer consists of several different species, in contrast to the simple ordered structures observed under high vacuum. After adsorption ethylene is also present as vinyl groups at high coverages in addition to ethylidyne (known from previous experiments). Acetylene is concluded to be dimerized and gradually hydrogenated. It desorbs, only in part, however, as a saturated product.
a
b
3000
2800
2800
cm
REFERENCES
Fig. 3. Spectra of adsorbed acetylene: (a) together with the gas phase after introduction of acetylene; (b) gas phase evacuated; (c) (a-b) difference spectrum; (d) after addition of 28 Pa hydrogen; (e) same as (d) after evacuation; (f) (d-e) difference spectrum.
on single-crystal surfaces [4,7]. Bands above 3000 cm-’ and at 1700,142O cm-l show that acetylene chemisorbs initially in unsaturated form. This is probably vinylidene and the high frequency of the C=C bond (1700 cm-‘) is caused by the Pt --) C=C electron transfer. Another possibility is that the vinylidene is dimerized and the high-frequency (in-phase) component of the interacting C=C vibrations is observed in the spectra (Fig. 4). This dimer is gradually hydrogenated, as is indicated by the strong methyl bands at 2970, 2880 and 1460 cm-l. Addition of hydrogen to samples covered with chemisorbed acetylene increased the intensity of the CH, band but evacuation restored the original spectrum Such cycles can be performed several times without significant degradation of the spectra (only a small amount of ethane is formed), indicating that strongly bound hydrogen-deficient species (carbon deposit) also exist on the surface, and that it can reversibly take up hydrogen but it cannot be saturated and desorbed completely. 3 c II Pt
N
v
c-c I Pt
\ Pt
""\ c-c
C"3 /
I\ I\ Pt Pt Pt Pt
3 3
7
C"3
/ CH-CH / Pt
\
Pt
Fig. 4. Possible surface reactions of chemisorbed acetylene.
1 L.L. Kesmodel, L.H. Dubois and G.A. Somorjai, J. Chem. Phys., 70 (1979) 2180. 2 P.C. Stair and G.A. Somorjai, J. Chem. Phys., 66 (1977) 573. 3 T.E. Fischer and S.R. Kelemen, J. Vat. Sci. Technol., 15 (1978) 607. 4 J.E. Demuth, Surf. Sci., 80 (1979) 367. 5 M.R. Albert, L.G. Sneddon, W. Eberhardt, F. Greuter, T. Gustaffson and E.W. Plummer, Surf. Sci., 120 (1982) 19. 6 H. Ibach and S. LehwaId, J. Vat. Sci. Technol., 15 (1978) 407. 7 H. Ibach, H. Hopster and B. Sexton, Appl. Surf. Sci., 1 (1977) 1. 8 E. Yagasaki and RI. Masel, Surf. Sci. 226 (1990) 51. 9 B.A. Morrow and N. Sheppard, Proc. R. Sot. London, Ser. A, 311 (1969) 391. 10 J.D. Prentice, A. Lesiunas and N. Sheppard, J. Chem. Sot., Chem. Commun., 76 (1976). 11 N. Sheppard and C. de la Cruz, React. Kinet. Catal. Lett., 35 (1987) 21. 12 Y. Soma, J. CataI., 59 (1979) 239. 13 T.P. Beebe and J.T. Yates, Jr., J. Phys. Chem., 91 (1987) 254. 14 T. Shibanuma and T. Matsui, Surf. Sci., 154 (1985) L215. 15 P.K. Wang, C.P. SIichter and J.H. Sinfelt, J. Phys. Chem., 89 (1985) 3060. 16 I.D. Gay, J. Catal., 108 (1987) 15. 17 T. Szilagyi, PhD Thesis, Institute of Isotopes of the Hungarian Academy of Sciences, Budapest, 1986. 18 R.J. Koestner, M.A. Van Hove and G.A. Somorjai, J. Phys. Chem., 87 (1983) 203. 19 H.A. Pearce and N. Sheppard, Surf. Sci., 59 (1976) 205. 20 J.D. Clewley, J.F. Lynch and T.B. Flanagan, J. Catal., 36 (1975) 291. 21 T. Szil&yi, J. Catal., 121 (1990) 223. 22 M. Procop and J. Viilter, Surf. Sci., 33 (1972) 69. 23 B.A. Morrow and N. Sheppard, J. Phys. Chem., 70 (1966) 2406. 24 N. Sheppard and J. Erkelens, Appl. Spectrosc., 38 (1984) 471.