J. Phys. Chem. 1986, 90, 4051-4057
405 1
cules, Fe(CO), and Fe(C5H5)*,listed in Table IV, are grouped into the translation-like, rotation-like, and vibration-like modes. The available degrees of freedom estimated on the basis of this table are given in Table 111.
mode belonging to the translation-like or rotation-like modes, only the vibrational mode with zero vibrational angular momentum, I = 0, is effective as the heat bath, so that such a degenerate mode should be counted as one (instead of two) available degree of freedom. The above prescription is applied to Fe(CO), and Fe(C,H,)2. The symmetry species of the normal modes of the parent mole-
Registry No. Fe(CO),, 13463-40-6; Fe(CSH&, 102-54-5; He, 744059-7;
Fe, 7439-89-6.
SURFACE SCIENCE, CLUSTERS, MICELLES, AND INTERFACES Oxldatlon of Sulfur Dioxide on Ag(ll0): Vlbratlonal Study of the Structure of Intermedlate Complexes Formed Duane A. Outka, R. J. Madix,* Departments of Chemical Engineering and Chemistry, Stanford University, Stanford, California 94305
Galen B. Fisher, and Craig DiMaggio Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090 (Received: September 6, 1985)
The vibrational spectra of intermediates formed in the low-pressure oxidation of SO2on the Ag(ll0) surface are investigated by electron energy loss vibrational spectroscopy. Sulfur dioxide first reacts with atomic oxygen on the Ag( 110) surface at 241 K to form a SO3 intermediate which is bonded to the surface by a single oxygen atom. This species irreversibly converts to a bidentate, oxygen-bonded species upon heating to 418 K. The SO3 species disproportionates at 500 K to yield gaseous SOz, adsorbed SO4,and subsurface oxygen. The SO4is initially formed as a bi- or tridentate, oxygen-bonded species with C, symmetry; it converts to a strictly bidentate species with C2, symmetry upon heating to 815 K. The SO4 decomposes above873 K.
Introduction The oxidation of sulfur dioxide on metal surfaces occurs during a variety of industrial processes including automobile emissions control, corrosion of metals, and poisoning of catalysts. While this reaction has been studied on a number of metals under catalytic conditions,' little has been done on well-defined surfaces and under low-pressure conditions. The adsorption of SO2 alone has been examined on several surfaces including W(polycrystalline),2 N i ( p ~ l y ) ,Rh( ~ 1 Pt( 110): Fe(poly) Pt( 11l),69' Ag( 11 0),&loAg( 111),loAg( 1OO)," and A ~ ( p o l y ) . ~Decomposition -~ occurs on all of these metals except Ag and Au. Sulfur dioxide adsorption on Ag( 110) has (1) Bond, G. C. Catalysis 6y Metals; Academic: New York, 1962; p 463. (2) Golub, S.; Fedak, D. G. Surf. Sci. 1974, 45, 213. (3) Brundle, C. R.;Carley, A. F. Faraday Discuss. Chem. SOC.1975,60, 51. (4) Ku, R. C.; Wynblatt, P. Appl. Surf. Sci. 1981, 8, 250. (5) Furuyama, M.; Kishi, K.; Ikeda, S . J. Electron Spectrosc. Relat. Phenom. 1978, 13, 59. (6) Koehler, U.; Wassmuth, H.-W. Surf. Sci. 1982, 117, 668. (7) Astegger, St.; Bechtold, E. Surf. Sci. 1982, 122, 491. (8) Outka, D. A.; Madix, R. J. Surf. Sci. 1984 137, 242. (9) Outka, D. A.; Madix, R. J.; Fisher, G. B.; DiMaggio, C. Langmuir, in press. (10) Rovida, R.; Pratesi, F. Surf. Sci. 1981, 104, 609.
been examined by a variety of techniques including temperature programmed desorption (TPD), low-energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and high-resolution electron energy loss spectroscopy (HREELS)9 which showed that SO2 adsorbs molecularly at pressures below 1 X lo-, Torr. Sulfur dioxide bonds to the Ag( 1 IO) surface via a weak Ag-S bond with a strength of approximately 64 kJ mol-'. Upon heating, sulfur dioxide desorbs completely. Fewer studies have examined the reaction of SO2 with oxygen on metal surfaces. On polycrystalline Ni3 and Fe5 surfaces, an SO4species has been identified by XPS from the reaction of SO2 with its dissociation products. On Pt( 11 l), SOz reacts with coadsorbed oxygen atoms to yield a trace of gaseous SO,, but largely an unidentified sulfur-oxygen-containing species which decomposes with heating to yield gaseous SO2 and oxygen. Oxidation on the Ag( 110) surface, in comparison, has been the most extensively studied. Strongly adsorbed sulfur- and oxygen-containing overlayers are formed following exposure of an oxidized Ag( 110) surface to sulfur dioxide. A similar reaction has been observed for silver powders exposed to SO2near ambient pressures.*," The reaction on Ag(l10) has been previously studied ~
~
~~
(1 1) Dorian, P. B.; von Raben, K. U.; Chang, R. K.; Laube, B. L. Chem. Phys. Lett. 1981, 84, 405.
0022-3654/86/2090-405 1$01.50/0 0 1986 American Chemical Society
4052
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
Outka et al.
TABLE I: Vibrational Frequencies and Assimments for SO,/Ag(llO) and SO3 Complexes freq, cm-I system
bonding'
M-S M-S M-S M-S M-S M-0 M-0, M-S M-0, M-S M-0, M-S
S-0 stretch
S-0 bend
885, 835 1180, 875, 805 1391, 1065 1010,961 1074, 986 1060, 977 1095, 1089, 1050 1060,9ao 1073, 1024, 975, 945 943, 8 15 1200, 980 1025, i o o a , 9 7 7 , 9 1 2 , 8 6 0 981, 894, 860 1035, 970, 908
558 558 536, 633, 646 633, 625, 645, 650, 638,
u(Ag-S03), em-'
ref
240 245
this work this work 38, 39 40 23, 24 25,41 21, 26 27 28, 41 41, 42 43,44 21 21 21
484 496
nr 518, 505
253
500, 475 517 630, 520 520
nr 366 252
nr
684,636,564,519,499,480 670,590,510,498,453 655,632,500,494,478
nr
nr nr
M-S = metal-sulfur bond; M-0 = metal-oxygen bond; M-0, M-S = both metal-sulfur and metal-oxygen bonding; nr = not reported.
TABLE II: Vibrational Frequeneies and Assignments for SOJAg(ll0) and SO4 in Complexes freq, cm-' system
bonding"
S-0 stretch
mono chel chel chel brdg brdg brdg, chel brdg
1275, 1070, 910 1285, 915 1106, 983 1170, 1095, 1030 1300, i i 7 0 , 8 a 5 , 8 6 0 1235, 1170, 1140, 920 1265, 1147, 892 1297, 1155, 955, 913 1280, 1160, 1105, io6o,ao8-a85 1300, 1200, 990,950, 760 1295, 1200, 1000
S-0 bend
u(Ag-SO,), cm-'
ref
605 610 622, 454
220 245
nr nr
nr nr nr nr nr nr nr
this work this work 38 45 46 47 31, 48 49 50 51 43,44
640, 600, 570 645 674, 652, 586, 501 695
nr
Mono = monodentate coordination; chel = chelating bidentate coordination; brdg = bridging bidentate coordination; and nr = not reported.
by XPS and TPRS (temperature programmed reaction spectroscopy), from which it was concluded that a reaction occurred between SO2and 0 near room temperature to form a surface SO3 intermediate. This intermediate disproportionated at 500 K to evolve SO2and form a surface SO, species and subsurface oxygen; no evolution of SO3 was observed. The SO, intermediate was stable to above 875 K where SO2 and O2were evolved. Thus on Ag( 110) SO2 shows profound reactivity changes in the presence of atomic oxygen. In this study we examine further the oxidation of sulfur dioxide on the Ag( 110) surface using HREELS. With this technique fhe vibrational spectra of the SO3and SO, species were examined for the first time on a well-defined surface. The SO3and SO, species both exhibit two different orientations on this surface, which are interpreted in terms of oxygen-bonded species and which have analogues in metal complexes. Overall, the adsorption of SO2 on clean and oxidized Ag( 110) surfaces is similar to that of C 0 2 adsorption on these surface^,'^-^^ although its reactions are more complicated, and its reactions further illustrate the nucleophilic behavior of oxygen adatoms on the Ag( 110) surface.
Experimental Section The experiments were performed in an ultra-high-vacuum (UHV) chamber described p r e v i o u ~ l ywith ~ ~ a base pressure of 2X TOK. The sulfur dioxide and oxygen were dosed through separate, collimated molecular beam dosers positioned 4 mm in front of the crystal. The exposures from these dosers were calibrated by comparing TPD peak areas to the peak areas from background exposures. Pressure measurements and exposures refer to uncorrected ion gauge readings. Sulfur dioxide exposures (12) Barteau, M. A,; Madix, R. J. J. Chem. Phys. 1981, 74, 4144. (13) Barteau, M. A,; Madix, R. J. J . Electron Spectrosc. Relat. Phenom. 1983, 31, 101. (14) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Chem. Phys. Lett. 1982, 89, 48. (15) Sexton, B. A. J . Vacuum Sci. Technol. 1979, 16, 1033.
were performed with a crystal temperature of 100 K. Oxygen doses were performed with a crystal temperature of 275 K followed by momentary annealing to 475 K to both ensure the absence of molecular oxygen and to order the atomic oxygen overlayer. The preoxidized surface was prepared by dosing 100 langmuirs (1 .O langmuir is equivalent to an exposure of 1 X 10" Torr for 1 s) of 02.The resulting coverage corresponded to approximately 2.1 X lOI4 atoms cm-2 or 0.25 monolayer of atomic oxygen (1 .O monolayer is defined as the number of surface silver atoms).I6 The energy losses were measured in the specular direction at 100 K after the surface was annealed momentarily to various temperatures. The incident beam energy was 3.5-4.0 eV.
Results Adsorbed SOz. The vibrational loss spectra for SOzadsorption on a preoxidized surface are shown in Figure 1 as a function of annealing temperature, and the mode assignments are summarized in Tables I and 11. The oxygen-treated surface alone showed a loss at 325 cm-' which is characteristic of atomic oxygen on Ag(1l0).l7 Following exposure to 1.0 langmuir of SOz, the annealing temperatures required to observe changes in the vibrational spectra qualitatively corresponded to the temperatures previously observed for changes in the XPS and TPRS,8 so the results were directly comparable. The spectra at 98 and 169 K have been previously discussed in detail;g the assignments for these will only by summarized. At 98 K the spectrum was typical of SOz multilayers. Losses were observed at 1315, 1145, and 535 cm-', corresponding well t o the spectrum of solid SOz multilayers, which has vibrational frequencies centered at 1320, 1147, and 525 cm-'.I8 Annealing to 169 K removed the multilayer, and new losses were observed corresponding to the spectrum of chemisorbed SOz. (16) Sexton, B. A.; Madix, R. J. Surf. Sci. 1981, 105, 177. (17) Backx, C.; deGroot, C. P. M.; Biloen, P. Surf.Sci. 1981; 104, 300. (18) Anderson, A.; Campbell, M. C. W. J. Chem. Phys. 1977,67,4300.
The Journal of Physical Chemistry, Vol. 90, No. 17, I986 4053
Oxidation of Sulfur Dioxide on Ag( 110)
I
C '
-C
'4
3 0
1000
2000
3000
4000
ENERGY LOSS (cm-')
-225
Y
610
M-SO4
8
r 1285
0
IOOC
2000
3000
4000
ENERGY LOSS (cm-'1 Figure 1. Electron energy loss spectra following an exposure first of 300 langmuir of O2 then 1.0 langmuir of SO2,both at 100 K. All spectra were recorded at 100 K after annealing to the indicated temperature. Spectrum at 98 K is of SO2multilayer; 169 K is of chemisorbed SO2on oxygen-treated surface; 241 K is of monodentate, oxygen-bonded SO,, 418 K is of bidentate, oxygen-bondedSO,;570 K is of SO4groups with C, symmetry: and 815 K is of SO4 group with C, symmetry.
The disappearance of the multilayer was inferred from the absence of the 1315-cm-' loss. The losses due to chemisorbed SO2on the preoxidized surface were observed at 1010, 670, and 565 cm-I. These corresponded roughly to the most intense losses of SO2 chemisorbed on the clean Ag(ll0) surface at 985,685, and 470 cm-'. Thus SO2and oxygen adatoms do not react below 169 K on the Ag( 110) surface. Surface SO3. Changes in the vibrational spectra that occurred between 169 and 241 K indicated that adsorbed SO2was oxidized.
Figure 2. Normal-mode vibrations of the SO, group according to ref 20. The vibrations are shown for the SO, group with C,, symmetry.
The atomic oxygen loss a t 325 cm-' was not observed after SO2 adsorption even at 95 K, presumably because of shadowing by the SOzoverlayer. While the absence of the loss for atomic oxygen could be taken as evidence for reaction of SO2 and oxygen, the similarity of the SO2spectrum at 169 K to that on the unoxidized surface suggested otherwise. Instead, new features appeared upon annealing to 241 K which were indicative of SO2oxidation. At 241 K the features of molecular SO2evolved into losses for a new species at 220, 585,835, and 885 cm-'. The latter two features were not fully resolved. A survey of the literature reveals that for all gaseous or complexed species of stoichiometry SO2,SO3, or SO4,the S-0 stretching frequencies are above 808 cm-' while the S-0 bending frequencies are below 700 cm-'. With these criteria the 835- and 8 8 5 - c d losses are assigned to S-0 stretches. These modes were too low in frequency to correspond to either of the S-0 stretching frequencies of an SO2species, however. For example, the asymmetric stretch for SO2 in complexes has not been observed below 1100 cm-'.19 Thus, in agreement with previous results* the vibrational spectra indicated that a species of new stoichiometry was formed, and the oxidation of SO2 occurred between 169 and 241 K. The changes in the vibrational spectra are consistent with the previous XPS assignment of the formation of a surface SO3species. Previous XPS studies have shown that the SO2 + 0 reaction produces an SO3surface species below 500 K. The vibrational spectra are in agreement with the existence of SO3on the surface, since the frequencies of the S-0 stretches at 835 and 885 cm-' were too low for SO4. For example, Table I1 shows that ionic sulfates and sulfate ligands have at least one and usually several frequencies above 1100 cm-I. In contrast, the frequencies at 835 and 885 cm-' are more reasonable for a species of SO3 stoichiometry (Table I). The vibrational modes for a pyramidal SO3 group are: vl (symmetric stretch), v2 (symmetric bend), v3 (asymmetric stretch), and v4 (asymmetric bend), according to the nomenclature of ref 20. These are illustrated in Figure 2. Under D3,, and C3, symmetry, the modes v, and v4 are both doubly degenerate, but as the symmetry of the SO3group is reduced these degeneracies are removed. Normal-coordinate analysis has not been performed for the various coordination geometries of the SO, group and so the precise assignment of all modes is difficult. The stretching and the bending modes of the SO, group, however, can be distinguished since the S-0 stretches for sulfites and gaseous (19) Ryan, R.R.;Kubas, G . J.; Moody, D. C.; Eller, P. G . Sfruct.Bonding 1981, 46, 47.
(20) Herzberg, G . Molecular Spectra and Molecular Structure II; Van Nostrand Reinhold: New York, 1945.
4054 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 TABLE I11 Vibrational Mode Representations for Adsorbed SO3 Belonging to Various Point Groups" point PrOUD c3,
C, CJ1)
Cs(2)
Cl C,
u,
u,
AI AI A' A' A A
AI B2 A' A" B A
Ud
U1
E
+
AI B, A ' + A" A ' + A' A+B A+A
E
+
AI B, A ' + A" A' A' A+B A+A
+
surface active s-0 s-0 stretch bend 1 2 2 3 2 3
1 1
2 2 1
3
C,( 1): The mirror plane symmetry element of this point group, u,, is located perpendicular to plane of oxygens of SO,. C3(2): SO, group is planar and parallel to u,,.
SO, are all observed above 800 cm-I, while the bends are observed only between 460 and 675 cm-1.21-23Thus the surface SO3group had at least two S-0 stretches at 835 and 885 cm-I and at least one bend at 585 cm-'. The loss at 220 cm-' is assigned to the Ag-(S03) stretch. The SO, on Ag( 110) was quite distinct from gaseous SO, which has a symmetric stretch, vl, at 1391 cm-' (Table 111). The v I vibration is surface dipole allowed in all point groups of less than D3,,symmetry (see below); a loss near this frequency would be observed if the SO, were adsorbed in a weakly bound molecular state. Therefore, this mode must be reduced in frequency to at least 885 cm-' in the surface S03.21 The low frequencies of the S-0 stretches a t 241 K indicated that a monodentate oxygen-coordinated SO3group was formed. The vibrational spectrum was distinct from the salt, Ag,S03, which has no infrared bands in the range 800-900 cm-I. Thus the formation of a bulklike Ag2S03layer is ruled out. These frequencies were also unlike those of ionic sulfites, such as Na2S03, with weak metal-oxygen and metalsulfur interactions which have a strong S-O stretch around 960-975 cm-1.21 Thus the existence of a covalent bond between the silver surface and the SO, group is inferred. Several types of bonding of SO3to metal atoms have been proposed from studies of sulfite complexes and sulfite salts.21 These include monodentate coordination via sulfur or oxygen and bidentate coordination via two oxygens or sulfur and oxygen. The best characterized coordination geometry is that of monodentate sulfur bonding for which several X-ray crystal structure determinations have confirmed the geometry.2e28 Table I shows the five S-bonded complexes for which X-ray studies have been performed, and in each case the S-0 stretching frequencies are greater than 945 cm-I, which significantly exceeds the S-0 stretching frequencies of SO, on Ag( 110). Nyberg and Larsson have generalized this behavior by noting that a strong mode above 975 cm-' indicates coordination through sulfur.21 The SO3 group on Ag( 110) did not exhibit such a peak, so monodentate bonding via sulfur is ruled out. Only one monodentate, oxygen-coordinated sulfite complex has been structurally characterized, and the S-0 stretching frequencies are much lower, 815 and 943 cm-I (Table I). The lowering of the S-0 stretching frequencies for the monodentate oxygen geometry can be explained by a simple Lewis electron structure model.2'.29 Two resonance structures can be drawn for the S032ion:
(21) Nyberg, B.; Larsson, R. Acta Chem. Scand. 1973, 27, 63. (22) Baldwin, M. E. J . Chem. SOC.1961, 2123. (23) Newrnan, G.; Powell, D. B. Spectrochim. Acta 1963, 19, 213. (24) Capporelli, M. V.;Becka, L. N. J. Chem. SOC.A 1969, 260. (25) Spinnler, M. A.; Becka, L. N. J. Chem. SOC.A 1967, 1194. (26) Baggio, S.;Becka, L. N. Acta Crystallogr., Sect. B 1969, B25, 946. (27) Dikareva, L. M.; Baranovskii, I. B.; Mekhtief, Z. G. Russ. J . Znorg. Chem. 1972, 17, 1772. (28) Porai-Koshits, M. A.; Ionov, S.P.; Novozhenyuk, Z. M. J . Struct. Chem. USSR 1965,6, 161. (29) Cotton, F. A.; Francis, R. J. Am. Chem. SOC.1960, 82. 2986.
Outka et al. 2-
2-
'2 II
El(-)
I
Attachments of a positive metal center to the sulfur atom should favor the first structure which has an average S-0 bond order of 1.33. Conversely, attachment of a positive metal center to an oxygen should favor the second structure with a lower average S-O bond order of 1. Thus monodentate oxygen bonding should have lower S-O stretching frequencies, and Nyberg and Larsson have suggested that complexes with strong S-0 vibrations below 960 cm-l indicate oxygen coordination. Since the surface SO, modes were below 960 cm-', this generalization suggests that SO3 on Ag(l10) at 241 K is bonded to Ag(l10) via a single oxygen atom. There are no structurally characterized examples of bidentate oxygen coordination to metals but dimethyl sulfite, CH,0S=OOCH3, has one strong S-O stretch at 1200 cm-I plus lower frequency S-0 stretching modes (Table I). The single high-frequency S-0 stretch of the bidentate oxygen coordinated SO3group can be explained with the first Lewis structure above. In this case, the SO, group is bonded by both of the oxygen atoms with formal negative charges, and the remaining S-0 bond is of order 2. The high-frequency S-0 stretch is thus associated with the S-0 bond of the uncoordinated oxygen. For a bidentate oxygen surface species the S-0 stretch of the uncoordinated oxygen would be surface dipole active; the lack of such a highfrequency loss in the HREELS a t 241 K thus suggests that the SO3 was not bidentate oxygen bonded to the surface. Bidentate coordination via sulfur and oxygen has been proposed for several complexes from chemical evidence and the number of infrared b a n d ~ . ~ I -No ~ , examples of discrete metal complexes with this geometry have been verified by X-ray diffraction; so it is not clear if this geometry really exists. The structures of several sulfite salts are known which have different metal atoms bonded to oxygen and sulfur (Table I). Nyberg and Larsson have summarized their vibrational behavior by noting that they have strong S-O stretches, both above and below 975 cm-1.21 In the absence of normal-coordinate analysis on systems of this geometry it is not possible to determine which modes would be allowed by the surface dipole selection rule in a surface spectrum. Thus we cannot definitively rule out this geometry, but the frequencies are rather unlike those of the surface SO3. In summary, the current understanding of sulfite coordination is that the SO, group is coordinated to Ag(ll0) by a single oxygen atom at 241 K. The apparent symmetry of the SO3group as determined from the number of observed S-0 stretching losses and application of the surface dipole selection rule is also consistent with monodentate oxygen bonding to the surface. The surface dipole selection rule stipulates that only those modes with a dipole moment component normal to the surface will be observed by HREELS in the specular direction. While this rule is not rigorously held for an HREELS experiment, it is a good approximation for those vibrational modes which exhibit strong dipolar absorption intensity in normal infrared spectroscopy. For example, the oxygen stretching modes of SO, and SO4, like those of CO and NO, are strongly dipole active and so should follow the surface dipole selection rule. An equivalent statement of the surface dipole selection rule is that only modes belonging to the totally symmetric representation will be observed. Whether the modes belong to the totally symmetric representation depends on the point group of the surface species. Table I11 shows the representation to which each vibrational mode belongs in each possible point group of a surface SO3. Table I11 also shows the number of surface dipole active stretches and bends expected for SO, as the symmetry is reduced from C, to CI (no symmetry). The surface SO, cannot have C,, symmetry because more than one S-0 stretch was observed. C3"symmetry would be possible only with sulfur coordination or for purely ionic bonding to the surface, both of which were also ruled out on the basis of the observed frequencies. The number of stretches and bends observed for the surface SO3exactly matches the number expected for C2,
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4055
Oxidation of Sulfur Dioxide on Ag( 1 10)
m OSS
(a)
@ F 2
Q
h4.09 (b)A -I
"2
"I
200-325 K 325-500 K Figure 3. Possible geometries for SO3group on Ag(ll0): (a) represents monodentate,oxygen-bonded species which is stable below 325 K while (b) represents bidentate, oxygen-bonded species stable between 325 and 500 K.
or C2 symmetry whereas an additional bending loss would be expected in the case of C,( 1) symmetry, and at least two extra losses are expected for Cs(2) or C I symmetry. These last three point groups cannot be rigorously eliminated by band counting, however, since the resolution of the data is insufficient to rule out the possibility of peak overlap. The frequency difference between SO3on Ag( 110) and neutral SO3further aids in the clarification of the symmetry of the surface SO, species. The difference in the stretching frequencies between neutral SO3and the surface species indicates that SO3on Ag( 110) was pyramidal, like a sulfite. This is supported by UPS studies which show electron donation to the lowest unoccupied molecular orbital of SO3,the 3 a F a s Thus, the twofold rotation axis and the mirror plane parallel to the plane of the oxygens of SO3@)are lost in S03(a), eliminating the C2,C,, and Cs(2) point groups. Thus the possible point groups for SO3on Ag( 110) are C,( 1) and C1. These point groups have one and three more modes than resolved in the spectra, respectively. The presence of an unresolved bending mode may have been consistent with the width of the 585-an-' loss, but the presence of three unresolved losses including an S-0 stretch was unlikely. Thus, the point group of SO3,formed by SO2oxidation, on Ag(ll0) at 241 K appears to be Cs(l). An example of this point group is illustrated in Figure 3a and it is consistent with a pyramidal SO3 group with monodentate oxygen bonding to the surface. Upon annealing to 418 K, a new loss appeared at 1 180 cm-l which is attributed to conversion of the monodentate SO,species to bidentate oxygen-coordinated SO,. This loss appeared at a temperature below that for conversion to SO4according to XPS8 and disappeared upon further heating to temperatures at which SO4did form. Thus its presence suggests either a change in the coordination of the surface SO3or a lowering of the symmetry of the SO,, thereby introducing a new loss. Comparison of the vibrational frequencies to those of monodentate sulfur-bonded sulfite complexes in Table I shows that this new frequency was 90 a n - I above that of any of these complexes, so this bonding mode is still unlikely. Only frequencies below 960 cm-I are expected for monodentate oxygen coordination, ruling this out as well. Frequencies up to 1250 an-'have been observed for the complexes proposed for bidentate coordination, so the SO3 may be converted to a bidentate surface coordinated As discussed above, the appearance of a high-frequency S-0 stretch would be expected for bidentate oxygen coordination as in C H 3 0 S = 0 0 C H 3 which has such a mode at 1200 cm-'. It is less obvious how the S-0 frequencies would change for S,O-bidentate coordination, and it is not possible to completely rule out S,O-coordination without better structurally characterized examples of such bonding. From available evidence, the spectrum at 418 K is most consistent with a bidentate oxygen-coordinated SO,. Surface SO,. Annealing to 570 K eliminated the vibrational losses of SO3and produced new losses which are attributed to an SO4species coordinated to the surface via more than one oxygen atom. Losses were observed at 220, 605, 910, 1070, and 1275 cm-I (Figure 1); the SO3losses at 835-885 and 1180 cm-' completely disappeared. Previous XPS studies have shown that conversion to an SO4species occurs above 500 K.* A vibrational frequency as high as 1300 cm-I is typical of variously coordinated SO4species (Table 11). The uncoordinated sulfate ion belongs
"3
Figure 4. Normal-mode Vibrations of the SO4group according to ref 20. The vibrations are shown for the SO4group with Td symmetry.
to the Td point group and has four fundamental vibrations: v 1 (symmetric stretch), u2 (symmetric bend), v3 (asymmetric stretch), and v4 (asymmetric bend) using the nomenclature of ref 20. With Tdsymmetry v2 is doubly degenerate, and v3 and v4 are both triply degenerate. These are illustrated in Figure 4. As the symmetry is reduced these degeneracies may be removed and additional vibrations observed. A normal-coordinate analysis has been performed for two of the coordination geometries of SO4: mond e n t a t e oxygen coordination and bidentate bridging coordinat i ~ n . Unfortunately, ~~ the difficulty of the normal-coordinate calculation for these systems required certain interactions to be neglected such as the coupling between ligand and metal vibrations. Thus the measured frequencies of known sulfate complexes vary widely, and it is difficult to precisely assign frequencies for other complexes based upon these calculations. Therefore, the vibrations of the surface SO4will only be assigned as either S-0 stretches or S-0 bends, which are widely separated in frequency. For example, comparing the frequencies of the surface SO4 to that of ionic sulfates and sulfate ligands shows that the loss at 605 cm-' is a bend while those at 910, 1070, and 1275 cm-I are S-0 stretches (Table 11). The loss at 220 cm-I is assigned to the Ag-(S04) stretch. Thus there were at least three S-0 stretches and one bend. Bonding via sulfur is impossible because of steric considerations, so all possible coordination modes involve only oxygen. Several possible sulfate coordination geometries have been proposed for complexes, including monodentate, chelating bidentate (both oxygens bonded to the same metal atom), bridging bidentate (two oxygen bonded to different metal atoms), and combinations of these. Only one monodentate complex has been structurally well characterized (Table II), but several others have been proposed from chemical and infrared e ~ i d e n c e . ~ ' These ,~~ show S-0 stretching frequencies in the range 970-1 170 an-1.31*32 The 1275-cm-' loss observed for the surface SO4was well above this range, making this geometry unlikely. Several chelating and bridging bidentate complexes with bonding involving two oxygen atoms have been structurally well characterized (Table 11), and such structures have also been proposed for other complexes based upon chemical evidence and infrared spectroscopy. These two geometries have modes at higher frequency, 800-1300 cm-1,31932 but cannot be distinguished from each other by vibrational spectroscopy. Since this frequency range is similar to that observed (30) Tanaka, N.;Sugi, H.; Fujita, J. Bull. Chem. Soc. Jpn. 1964,87,640. (31) Valentine, J.; Valentine Jr., D.; Collman, J. P. Inorg. Chem. 1971, 10, 219. (32) Horn, R.W.; Weissberger, E.; Collman, J. P. Inorg. Chem. 1970, 9, 2361.
4056 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
Outka et al.
TABLE IV: Vibrational Mode Representations for Adsorbed SO, Belonging to Various Point Groups point group Td
G" Czu C,
C,
surface active PI
v3
"2
Ai E AI E Ai AI A2 A' A ' + A" A A+A
+
F2
A,
v4
stretch bend
F2
+E
+ +
Ai B, B, A ' + A ' + A" A+A+A
Ai + E AI BI B2 A ' + A ' + A" A+A+A
+ +
2 2 3 4
1 2 3 5
for SO4on Ag( 110) at 570 K, we conclude that S04(a) is bonded to the surface via more than one oxygen atom. The higher frequency of the S-0 stretches for the bidentate complexes vs. the monodentate complexes can be explained by a simple Lewis electronic structure argument. The sulfate anion has the following Lewis structure:
IO
bl
2-
T g
With monodentate coordination via one of the formally negative oxygens, the average S-0 bond order of the remaining S-O bonds is 1.66. In contrast, with two of the formally negative oxygens attached to metal centers, the average S - 0 bond order of the remaining S-O bonds is 2.0. Thus higher S-O bond strengths are expected for bidentate coordination resulting in higher S-0 stretching frequencies. The symmetry deduced for the SO, from the number of S-O stretching losses is C, or less, which is consistent with multiple oxygen atoms bonded to the surface. Table IV shows the number of surface dipole allowed stretches and bends as the symmetry is reduced from C,, to C, (no symmetry). This shows that the surface SO4cannot have C,, or C, symmetry because three S-O stretching losses were observed whereas only two are allowed in these point groups. The C3,point group is also impossible because it can only occur with monodentate oxygen coordination which was eliminated by the observed frequencies. The C, point group has exactly the number of stretches observed but predicts the observation of three nondegenerate bends, whereas only one was observed. Like SO3,all of the bends of SO4 may not be resolved in HREELS because of the limited resolution and the small frequency range in which they occur. Furthermore, since this region is usually obscured in infrared spectra of sulfate complexes, it is not certain that all of these modes are of sufficient intensity to be observed. The C, point group predicts four nondegenerate S-O stretches whereas only three were observed. Still, this cannot be rigorously eliminated because of the limited resolution. The C, point group is appealing since it is consistent with multiple oxygen bonding and can easily be formed, for example, from a tilted bidentate SO, group, which brings a third oxygen near the surface (Figure sa). Annealing the surface to 815 K led to loss of the peak at 1070 cm-', indicating conversion of the SO4 to a higher symmetry configuration, belonging to the C, point group. This form was exclusively formed at low oxygen predoses (20 langmuirs). This high-temperature form exhibited two S-O stretches, which according to the surface dipole selection rule is consistent with C,, or C, symmetry (Table IV). C, is only possible with monodentate coordination, however, which is inconsistent with the presence of the 1285-cm-' loss (see above). C, symmetry is consistent with strict bidentate coordination where the two uncmrdinated oxygens are equivalent. A simple motion of the SO., group to the vertical could affect the HREELS change between 570 and 815 K (Figure 5b). Upon heating to 873 K the losses of SO4 disappeared and the vibrational loss spectrum characteristic of the clean surface resulted. In the previous XPS study of this system, subsurface oxygen was observed as a product which was not detected here by HREELS. Previous attempts to study subsurface oxygen by HREELS have shown that it is not observable by this techniq~e.'~
ia I 500-800 K
(bl
800- 850 K Figure 5. Possible geometries for SOpgroup on Ag(ll0): (a) represents the SO4 group which is stable between 500 and 800 K with C, symmetry while (b) represents the SO4groups which is stable between 800 and 850 K with C , symmetry.
Discussion The vibrational spectra of the SO3 and SO4 intermediates indicate that they are attached to the Ag( 110) surface via oxygen which resembles the configuration of C03, formate (HC02), and methoxy (CH30) on this surface. This configuration differs from that of SO2, CO, and NO on metals where the oxygens are directed away from the surface.33 While binding sites cannot be definitively determined from vibrational data alone, several pieces of evidence suggest that the oxygens of the surface SO, and SO4intermediates are attached to the surface at the long twofold bridge site. Structural studies of two other oxygen-bonded surface species, methoxy and formate on copper surfaces,34indicate that oxygen bound species prefer higher coordination sites which differ only slightly from the adsorption site of atomic oxygen. If SO,and SO, follow this trend, they attach to the surface via oxygens located in the long twofold bridge site, by analogy to atomic oxygen on Ag(1 10)35-36 (Figures 3 and 5 ) . This configuration is consistent with several aspects of the vibrational spectra. For example, the sulfur atom of the SO,is sufficiently distant from any Ag atom to prevent Ag-S bonding, in agreement with the frequency analysis presented above. In contrast, if the oxygen of a monodentate SO,chooses to bind a t a hollow position above a second layer atom, then a small tilt of approximately 30" would bring the sulfur close enough to a silver atom to give a Ag-S distance similar to that of sulfur-bonded SO3complexes of p a l l a d i ~ m . ~Since ~ ~ * the ~ vibrational frequencies do not support the presence of such a Ag-S interaction, and since oxygen adatoms do not choose such a position, the hollow site is considered unlikely. With the oxygens of the SO3 located in the long twofold bridge site, the transition from a monodentate SO3 to a bidentate SO3 could be accomplished by a simple tilt of the SO, to place a second oxygen in a neighboring long twofold bridge site (Figure 3). The 0-0separation in SO,complexes is typically 2.4 A whereas the separation between the long twofold bridge sites on Ag(ll0) is 2.89 A, so a fair match is obtained. With a Ag-0 bond length of 2.2 8, derived from the structurally characterized 0-bonded iron-sulfite complex and a correction of 0.18 A for the different atomic radii of Fe and Ag, the Ag-S distance is 4.0 A which is much longer than the metal-sulfur bond in sulfur-bonded palladium sulfite complexes of 2.25 A. Thus this position for the bidentate SO3 also eliminates any Ag-S bonding, in agreement with the vibrational analysis of this study. Of course, a site on top of the atomic ridges of the (1 10) surface would also prevent any metalsulfur interaction but bonding in this site does not follow the preference of oxygen-bonded surface molecules for sites of high coordination. By the same reasoning the SO, is proposed to form bidentate oxygen bonds to the surface by spanning between adjacent long twofold bridge sites (Figure 5 ) . In this case a third oxygen can interact with ridge atoms by a simple tilt of the molecule. This geometry is consistent with the low-temperature vibrational spectra (33) Recent investigations have found evidence for a horizontal orientation for CO on the surface of Cr(ll0): Shinn, N. D.; Madey, T. E. Phys. Reu. Lett. 1984, 53, 2481. (34) St6hr. J.; Outka, D. A.; Madix, R. J. Surf.Sci. 1985, 164, 235. (35) Heiland, W.; Iberl, F.; Taglauer, E.; Menzel, D. Surf.Sci. 1975, 53, 787
(36) Heiland, W. Appl. Surf.Sci. 1982, 13, 282
J . Phys. Chem. 1986, 90,4057-4063
4057
bridge sites of 2.89 A. The SO3 also differs from the C 0 3 in that for the surface SO4,and it has C, symmetry (Figure sa). At higher it disproportionates to form an SO4 species whereas the C 0 3 temperatures an upright configuration with C,, symmetry is merely dissociates to form gaseous C02and adsorbed oxygen. This suggested with no third Ag-0 interaction (Figure 5b). Again, difference is attributable to the inaccessibility of the +6 oxidation a site on top of the atomic ridges of the (1 10) surface is also state for carbon. consistent with the symmetry and the frequency assignments of this study, but this would result in a lower coordination of the Acknowledgment. We (D.A.0 and R.J.M.) gratefully acoxygen atoms bound to the surface which we feel is unlikely. knowledge the support of the National Science Foundation (CPE The reaction of SO2 with atomic oxygen on Ag( 110) parallels 8320072) for this work. The cooperation of the management of the reaction of C 0 2with atomic oxygen on this same s ~ r f a c e . ' ~ - ~ ~the General Motors Research Laboratories in facilitating this In both cases a trioxide species is formed near 200 K, and the research is also deeply appreciated. structure of both species has been interpreted in terms of monRegistry No. SO2, 7446-09-5; Ag, 7440-22-4. odentate oxygen coordination. The SO3 differs in that an irreversible transformation to a bidentate species is observed, whereas (43) Detoni, S.;Hadzi, D. Spectrochim. Acta 1957, 11, 601. no such transformation occurs for the C03. One could speculate (44) Pauchert, C. J. The Aldrich Library of Infrared Spectra; Aldrich that the smaller size of the C03with its typical 0-0separation Chemical Co.: Milwaukee, WI, 1981. (45) Balvich, J.; Fivizzani, K. P.; Pavkovic, S. F.; Brown, J. N. Inorg. of 2.15 A3' is a poorer match for the span between long twofold (37) Wyckoff, R. W. G. Crystal Structures; Interscience: New York, 1964; Vol. 2, p 365. (38) Stopperka, K. Z. Anorg. Allg. Chem. 1966, 345, 277. (39) Kalder, A,; Maki, A. G.; Dorney, A. J.; Mills, I. M. J. Mol. Spectrosc. 1973, 45, 247. (40) Evans, J. C.; Berstein, H. J. Can. J . Chem. 1955, 33, 1270. (41) Hall. J. P.: Griffith. W. P. Inorp. Chim. Acta 1955. 48. 1270. (42) Larsson, L. 0.; Niinisto, L. AcG Chem. Scand. 1973, 27, 859.
Chem. 1976, 13, 71. (46) Reed, J.; Soled, S.L.; Eisenberg, R. Inorg. Chem. 1976, 15, 71. (47) Benelli, C.; Di Vaira, M.; Nocvidi, G.; Socconi, L. Inorg. Chem. 1977, 16, 182. (48) Lucas, B. C.; Moody, D. C.; Ryan, R. R. Cryst. Struct. Commun. 1977, 6, 57. (49) Muravieskaya, G. S.; Kukina, G. A.; Orlova, V. S.; Eustafeva, 0. N.; Porai-Koshits, M. A. Dokl. Akad. Nauk, SSSR 1976, 226, 76. (50) Cotton, F. A.; Frenz, B. A.; Shive, L. W. Inorg. Chem. 1975,14,649. (51) Ghatak, I.; Mingos, D. M. P.; Hursthouse, M. B.; Abdul Malik, K. M.; Transition Met. Chem. (N.Y.) 1979, 4, 260.
Metal-Adsorbate Vibrational Frequencies as a Probe of Surface Bonding: Halides and Pseudohalldes at Gold Electrodes Ping Gao and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received: November 21, 1985; In Final Form: March 13, 1986)
The frequencies of metal surface-adsorbate vibrations for monolayers of chloride, bromide, iodide, thiocyanate, and cyanide adsorbed at gold electrodes as obtained by surface-enhanced Raman scattering are compared along with corresponding data for silver electrodes and for related bulk-phase metal complexes with the aim of elucidating the nature of the surface bonding. The frequencies and derived force constants are generally substantially higher at gold than at silver, especially when the comparisons are made at the same metal charge rather than at a common electrode potential. This is interpreted in terms of a greater degree of bond covalency, Le., adsorbate-surface charge transfer, at gold. Also consistent with this interpretation are the derived force constants for gold that increase from chloride to iodide and the frequency-potential dependencies that decrease in the same sequence. The latter can be understood in terms of the influence of the electrode charge on the ionic component of the bond energy. Greater surface-adsorbate frequencies and smaller frequency-potential dependencies are found also for thiocyanate at gold compared with silver electrodes, again implicating the greater importance of bond covalency at the former surface. Some limitations of 'surface complex" models for describing SERS for such simple anion adsorbates at gold are also noted.
A primary application of surface vibrational spectroscopies at metalsolution as well as other types of interfaces is to gain insight into the nature of the metal-adsorbate bonding. While most attention has been focused on vibrational frequency shifts of internal adsorbate modes upon surface binding, the frequencies of the surfaceadsorbate modes themselves are clearly of particular significance. The paucity of information on the latter is probably due in part to the difficulties in detecting such low-frequency modes from electron energy loss or infrared spectroscopies. Nevertheless, surface Raman spectroscopy can readily detect such modes, even for surface-adsorbate bonds involving heavy atoms where their frequencies should be below ca. 300 cm-I. Of particular electrochemical interest are surface-adsorbate interactions for halides and pseudohalides in view of their structural simplicity and strong adsorption at metal surfaces. A significant amount of information on surface-adsorbate vibrations for these systems has been accumulated for the silveraqueous inteface using
surface-enhanced Raman scattering (SERS).1,2 Similar reports for the other SERS-active surfaces, gold and copper, have been sporadic and usually less reliable (vide infra). However, we have recently reported an ex situ electrochemical roughening procedure for gold that yields unusually stable as well as intense SERS.3 (1) For example: (a) Wetzel, H.; Gerischer, H.; Pettinger, B. Chem. Phys. Lett. 1981, 78, 392. (b) Fleischmann, M.; Hendra, R. J.; Hill, I. R.; Pemble, M. E. J. Electroanal. Chem. 1981, 117, 243. (c) Owen, J. F.; Chen, T. T.;
Chang, R. K.;Laube, B. L. Surj. Sci. 1983, 125, 679. (d) Weaver, M. J.; Hupp, J. T.; Barz, F.; Gordon, J. G.; Philpott, M. R. J. Electroanal. Chem. 1984, 160, 321. (2) For example: (a) Wetzel, H.; Gerscher, H.; Pettinger, B. Chem. Phys. Lett. 1981,80, 159. (b) Weaver, M. J.; Barz, F.;Gordon, J. G.; Philpott, M. R. Surf. Sci. 1983,125,409. (c) Dornhaus, R.; Long, M. B.; Benner, R.E.; Chang, R. K. Surf. Sci. 1980, 93, 240. (d) Fleischrnann, M.; Hill, I. R.; Pemble, M. E. J. Electroanal. Chem. 1982, 136, 361. (3) Gao, P.; Patterson, M. L.; Tadayyoni, M. A.; Weaver, M. J. Lungmuir 1985, I , 173.
0022-3654/86/2090-4057$01 .SO10 0 1986 American Chemical Society
