Langmuir 1989,5, 123-128 coordination site, designated as an "outer" or "weaker" site, and hydrogen-bonded complexes are identified. Brernsted acid coordination is detected only when a strongly basic adsorbate, n-butylamine, is employed as the probe molecule. No discrimination between the concentration of Lewis acid sites on 6-6 and K aluminas is made by either pyridine or acetonitrile. The strength of the sites, as quantified with pyridine, is similar; however, with acetonitrile, a stronger coordination complex is formed on K alumina than on 6-8 alumina. The environment surrounding the acid site affects the adsorption of 2,6-lutidine; although it is a stronger base than pyridine and should, therefore, interact more strongly with alumina, only a weakly bound species is formed (see Tables I and 11). Although no intrinsic structural property differentiates the transition a l ~ m i n a sit, ~has ~ been suggested that an important variable is hydroxyl content.52 A major dif-
123
ference between the two aluminas is the extent of hydrogen-bonded species formed with an adsorbate. With larger adsorbates, pyridine, 2,6-lutidine, and n-butylamine, more hydrogen-bonded complexes occur on K alumina than on 6-8 alumina. With the smallest adsorbate studied, acetonitrile, there are more hydrogen-bonded species on 6-6 alumina than on K alumina, suggesting that the pore openings of the former are smaller than those of the latter.
Acknowledgment. The assistance of Dr. John W. Novak, Jr., Dr. Meg Martin Thompson, and Raymond Colbert is gratefully acknowledged. M.H.H. thanks the Exxon Education Foundation for support. Registry No. AlzOs, 1344-28-1; acetonitrile, 75-05-8; 2,6lutidine, 108-48-5;pyridine, 110-86-1;n-butylamine, 109-73-9. (52)Stone, F. S.;Whalley, L. J. Catal. 1967,8, 173.
Structure and Composition of Pt(ll1) and Pt(100) Surfaces as a Function of Electrode Potential in Aqueous Sulfide Solutions Nikola Batina, James W. McCargar, Ghaleb N. Salaita, Frank Lu, Laarni Laguren-Davidson, Chiu-Hsun Lin, and Arthur T. Hubbard* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 -0172 Received May 2, 1988. I n Final Form: August 15, 1988 Studies are reported in which surface layers formed by immersion of well-defined Pt(ll1) and Pt(100) electrode surfaces into aqueous NazS solutions were characterized with regard to structure, composition, and reactivity by means of low-energy electron diffraction (LEED),Auger electron spectroscopy, electron energy-loss spectroscopy (EELS), linear scan voltammetry, and coulometry. Voltammetry reveals that only oxidative desorption of S occurs on the Pt surfaces; no S reductive desorption is observed over the useful potential range. Combined surface analysis data (Auger),vibrational spectra (EELS),and structural data (LEED) permit identification of adsorbed layer composition and structure on the Pt(ll1) and Pt(100) surfaces as a function of potential. At potentials between -0.6 and 0.0 V (vs Ag/AgCI), LEED reveals that stable ordered adsorbed sulfur layers are formed on both surfaces: Pt(lll)(d3Xd3)R3O0-S and Pt(100)(d2Xd2)R45°-S. The best clarity of the LEED patterns is found at pH 9. Potentials more positive than 0.0 V give rise to increasingly diffuse intensity related to oxidative desorption of S. Voltammograms for oxidative desorption of S from both surfaces are markedly different, indicating different mechanisms of S oxidation at the two surfaces: at pH 9, four voltammetric peaks are present for S at the Pt(ll1) surface, compared with only one peak for the Pt(100) surface. Coulometric data reveal that approximately six electrons are transferred in oxidation of adsorbed S at both surfaces at pH less than 10. Voltammetric behavior of the sulfur layer is sharply dependent upon pH.
Introduction In previous papers, we reported that ordered layers, adlattices consisting primarily of uncharged adsorbed species, are formed when well-defined Pt(ll1) and Pt(100) surfaces are immersed into aqueous ionic solution^.^-^ Several aspects of surface behavior in these adlattice systems have been investigated, such as relative retention affinities of cations?' pH and potential dependencies of (1) Sticknev, J. L.; Rosasco, S. D.; Salaita, G. N.: Hubbard, A. T. Langmuir 1985, 1, 66. (2)Salaita, G.N.;Stern, D. A.; Lu, F.; Baltruschat, H.; Schardt, B. S.; Stickney, J. L.; Soriaga, M. P.; Frank, D. G., Hubbard, A. T. Langmuir 1986. 2. 828. (3) Stern, D. A.; Baltruschat, H.; Martinez, M.; Stickney, J. L.; Song, D.; Lewis, S. K.; Frank, D. G.; Hubbard, A. T. J.Electround Chem. 1987, 217. 101. (4) Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T. J. Electroanal. Chem. 1987,222, 305. ~~
(5)Salaita, G. N.;Lu, F.; Laguren-Davidson, L.; Hubbard, A. T. J. Electroanal. Chem. 1987, 229, 1.
0743-7463/89/2405-0123$01.50/0
electrochemical adsorption-desorption p r o c e s s e ~ ,and ~*~ metal electrodeposition at electrodes with a well-characterized surface adlatti~e."'~ Such studies are found to reflect the structure and intermolecular interactions within (6)Rosasco, S. D.; Stickney, J. L.; Salaita, G. N.; Frank, D. G.; Katekaru, J. y.;Schardt, B. C.; Soriaga, M. P.; Stern, D. A.; Hubbard, A. T. J. Electroanal. Chem. 1985,188,95. (7)Frank, D. G.;Katekaru, J. Y.; Rosasco, S. D.; Salaita, G. N., Schardt, B. C.; Soriaga, M. P.; Stern, D. A.; Stickney, J. L.; Hubbard, A. T. Langmuir 1985,1, 587. (8) Stickney, J. L.; Rosssco, S. D.; Song, D.; Soriaga, M. P.; Hubbard, A. T. Surf. Sci. 1983, 130, 326. (9)Stickney, J. L; Rosasco, S. D.; Hubbard, A. T. J. Electrochem. SOC. 1984,131, 260. (10)Stickney, J. L.; Stern, D. A.; Schardt, B. C.; Zapien, D. C.; Wieckowski, A.; Hubbard, A. T. J.Electroanal. Chem. 1986, 213, 293. (11)Stickney, J. L.; Schardt, B. C.; Stern, D. A.; Wieckowski, A.; Hubbard, A. T. J.Electrochem. SOC. 1986, 133, 648. (12)Schardt, B. C.;Stickney, 3. L.; Stern, D. A.; Wieckowski, A.; Zapien, D. C.; Hubbard, A. T. Langmuir 1987, 3, 239. (13)Schardt, B. C.;Stickney, 3. L.; Stern, D. A.; Wieckowski, A.; Zapien, D. C.; Hubbard, A. T. Surf. Sci. 1986, 175, 520.
0 1989 American Chemical Society
124 Langmuir, Vol. 5, No. 1, 1989 the lattice. Evidence for strong gas-phase adsorption of S on platinum single crystals from UHV has been reported.'*-l6 In the present article we report studies of the adsorption and electrochemical oxidation of sulfur/sulfide species at well-defined Pt(ll1) and Pt(100) surfaces in aqueous NazS solutions. Surface and adsorbed layer structures were examined by means of LEED. Surface elemental composition was investigated with Auger electron spectroscopy. Surface vibrational spectra were obtained by use of EELS. The results of these studies reveal that adsorption of S2from aqueous solution leads to formation of a highly ordered, uncharged layer of sulfur atoms at Pt(100) and Pt(ll1) over a wide range of electrode potentials. Of course, interaction of sulfide ions with platinum and other metals is of fundamental interest as an example of strong adsorption of anions and is of applied interest in connection with passivation of metal surfaces, poisoning of catalysts, behavior of fuel cell anodes, and other practical situations. This is the first time that the structure, valency, and composition of electrosorbed sulfide layers have been experimentally observed.
Experimental Section Instrumentation and procedures employed have been described in ref 1-5. The Pt(100) and P t ( l l 1 ) single crystals used in this work were oriented and polished such that all six faces of each crysid were crystallographically equivalent to the (100) and (111) planes, respectively. Both crystals were present in the instrument at the same time but were electronically insulated from each other during the electrochemicalexperiments, as the two surfaces display differing energetics and kinetics. All six faces of each crystal were cleaned simultaneously by bombardment with Ar+ ions and were annealed by resistance heating (about 1000 K) under ultrahigh vacuum. After characterization of both surfaces by low-energy electron diffraction (LEED) and Auger electron spectroscopy, the crystals were isolated in an argon-fid antechamber for immersion simultaneously but independently into various electrolytic solutions. Electrode potentials and currents were measured and controlled by means of standard electrochemical circuitry based on operational amplifiers. Potentials were referred to a Ag/AgCl reference electrode prepared with 1M KCl. The electrochemical cell was constructed of Pyrex glass and Teflon. After the solution was drained away, a film of liquid about lo4 cm thick remained on the crystal(s). The water layer evaporated a t the start of evacuation of the Ar atmosphere. Solutions employed for the linear scan voltammetric and coulometric experiments contained at least 10 mM supporting electrolyte in order to provide adequate conductivity and buffering capacity. However, prior to LEED, EELS, and Auger experiments, the surface was rinsed with 0.1 mM NaF (pH 9) at the same potential in order to minimize the contribution due to excess salts present in the liquid film upon removal of solution. All solutions used in this study were prepared from water pyrolytically distilled in pure oxygen through a Pt gauze catalyst. Aqueous sulfide solutions were prepared from pyrolytically distilled water which had been boiled and deaerated with oxygen-free nitrogen to prevent air-oxidation of sulfide. Solutions were continually purged with nitrogen. The Pt(ll1) and Pt(100) crystals were immersed a t constant potential (-0.6 V vs Ag/AgCl) for 3 min in a solution of 0.1 mM NazS containing 10 mM NaF, adjusted to pH 9 with 10 mM NaOH. The electrodes were subsequently rinsed several times with 10 mM electrolyte (pH 9), and the potential was stepped to more positive potentials for 300 s. Finally, the electrode was rinsed several times with a dilute NaF solution (pH 9) a t the stepped potential. (14) Berthier, Y.; Pedereau, M.; Oudar, J. Surf. Sci. 1973, 36, 225. (15) Heegemann, W.; Meister, K. H.; Bechtold, E.; Hayek, K. Surf. Sci. 1975, 49, 161.
(16) Hayek, D.; Glassl, H.; Guttmann, A.; Leonhard, H.; Prutton, M.; Tear, S. P.; Welton-Cook, M. R. Surf. Sci. 1985, 132, 419.
Batina et al.
&
DECREASED
.
B-
SENSITIVITY a 4
\
5 -I s
Pt
.
A
Pt 100
,
300
.
I
,
500
700
KINETIC ENERGY (eV) Figure 1. Auger spectra. (A) Clean Pt(ll1). (B) P t ( l l l ) ( d 3Xd3)R30°-S formed by immersion of Pt(ll1) into 10 mM N a a (pH 9) a t -0.6 V vs Ag/AgCl (1 M KCl). (C) Pt(lll)(diffuse)OH/O, resulting from oxidation of the surface in B at 0.8 V for 180 s in 10 mM NaF (pH 9). Experimental conditions: incident beam normal to the surface, 100 A,2000 eV; modulation, 5V p p . Oxygen packing densities are measured by means of Auger spectroscopy, with use of eq 1, with Bo = 1.272 cm2/nmol a t P t ( l l 1 ) or 0.994 a t Pt(100)
ro= ( I ~ / I ~ ~ O ) / B ~
(1)
where Ip: is the Pt Auger signal a t 235 eV from the clean Pt surface.
Results and Discussion Sulfide at Pt(ll1). Auger spectra of clean P t ( l l l ) , of the Pt(lll)(d3xd3)R30°-S adlattice, and of the surface after oxidative desorption of sulfur are shown in Figure 1. Even when the sulfur layer is prepared in electrolyte containing 10 mM KF and 0.1 mM K2S, followed by rinsing with 0.1 mM KF (pH 9), no detectable K is present. The limit of detection of K by our Auger spectrometer is about 0.002 nmol/cm2. Since these conditions would have resulted in retention of K+ ions if the surface layer had contained adsorbed anions,? the absence of a K Auger signal is evidence that the layer formed by adsorption from S2- solution is nonionic:
-
S2- + Pt PtS + H+ + 2e(2) LEED after immersion of Pt(ll1)into S2- solutions at pH 9 yields straightforward Pt(lll)(d3Xd3)R3O0-S patterns, as shown in Figure 2A. This same LEED pattern is found at all S2- concentrations and at all electrode potentials at which the S layer is stable (potentials less positive than 0.00 V vs Ag/AgCl, 1M KC1 reference). The structure corresponding to this simple LEED pattern is shown in Figure 3A. The ideal packing density of S in this structure is Os = 1/3 sulfur atom/surface platinum (rs = 0.83 nmol/cm2). Maximum sharpness of this LEED pattern occurs at -0.6 V. Accordingly, the S layer formed at -0.6 V is taken as the packing density reference point. This leads to Bs = 9.25 cm2/nmol in eq 3: rs = ( W I P t O ) / B S (3) where Ip2is the Pt Auger signal at 235 eV for the clean Pt(ll1) surface. Background contributions to the S Auger signal at 149 eV due to a nearby Pt Auger peak at 161 eV were digitally subtracted.
Structure and Composition of Pt Surfaces
Langmuir, Vol. 5. No. 1. 1989 125
Figure 2. LEED patterns after immersion of Pt(ll1) and Pt(100) into aqueous Na,S solutions. (A, Left) Pt(111)(d3Xd/3)R3O0-S at 68 eV. (B,Right) Pt(100)(dZxdZ)R45"-S at 41 eV. Experimental conditions: electrode potential, 4 . 6 V; adsorption from 10 mM NaF. 0.1 mM Na,S a t pH 9; rinsing with 0.1 mM NaF (pH 9).
B WTENTIAL.
WLT n NAPCI
POTENTI&.
MLI
-
llOa6'
Figmm 4. Packing densitiesof S adsorbed at Pt(ll1)and pUl00) versus electrode potential. (A) pU111). (B)PUlW). nplimental conditions: adsorption from 0.1 mM N a a in 10 mM NaF (pH 9)at 4.6 V; rinsing with 10 mM NaF (pH 9) at -0.6 V followed by rinsing at the potential indicated in the graph for 300 s and further rinsing, with 0.1 mM NaF (pH 91, prior to evacuation. Pi urn 3. Model structures. (A) Pt(lll)(d3Xd3)R3O0-S. = y/s s atom/surtace Pt atom; rs = 0.830 nmol/cmz. (B)P t (100)(d'2Xd/2)FU5°-S.Bs = '12 S atom/surfacePt atom; rs =
1.08 nmol/cm2.
Sulfur packing densities measured by means of Auger spectroscopy over a range of electrode potentials are given in Table I and Figure 4A. Also, shown are the oxygen packing densities. Desorption of S at positive potentials
is accomplished by adsorption of oxygen-containing species, as can be seen from the Auger data. Cyclic voltammetry is shown in Figure 5 (first scans, starting from the open circuit potential). scanning initially in the negative direction (dashed curves), followed by Auger spectroscopic characterization of the surface, revealed no reductive desorption of S from the surface. However, manning in the positive direction (solid curves) led to removal of S from the surface as judged by Auger
Batina et al.
126 Langmuir, Vol. 5, No. 1, 1989
2
-30l -08 -06 - C 4 - 0 2
D3-EhT L L
Of 0 2 0 4 0 6 0 8 t3-T
"
'
'
- 0 8 -06
80
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0 8 -06 - 0 4 - 0 2 0 0 0 2
F9TEN.I;lA.L.
5 5
N
\
I
'
00 0 2
POTENTIAL,
Ag/bqCl
'
'
'
-04 -02
'
04 06 VI
'
08
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Ap/4gCI
04 06
08
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VOLT i s Cg/AgCI
50 40
30 20 IO
$ 0 W
0 -10
+
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z W
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o 3
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.. -10 -08 -06 - 0 4 - 0 2
POTENTIAL,
3
08 -36
04
POTENTIAL
02
00 0 2 0 4
WLT
06
00 0 2 04 0 6 0 8
VOLT "I A ~ / A ~ C I
08
VI ag/tqc
Figure 5. Cyclic voltammetry of adsorbed S at Pt(ll1) and Pt(100). Pt(111)(d3Xd3)R30°-S a t (A) pH 9; (B) pH 9.5; (C) pH 10; (D) pH 12; (E) pH 14. Pt(lOO)(d2xd2)R45"-S a t (F) pH 9; (G)pH 9.5; (H) pH 10; (I) pH 12; (J)pH 14. Solid curve: immersion into 0.1 mM N e S , 10 mM NaF (pH 9), followed by rinsing with electrolyte and scanning positive from the open circuit potential. Dashed curve: negative-going scan under same conditions. Dotted curve: positive-going scan from the open circuit potential in pure electrolyte. Experimental conditions: scan rate, 5 mV/S; electrolyte, 10 mM NaF adjusted to given pH with NaOH (A, B, C, F, G, H); 10 mM NaOH (D, I), 1 N NaOH (E, J); temperature, 23 A 1 "C; reference electrode, Ag/AgCl (1 M KCl).
spectroscopy following oxidation. Hydrogen adsorption peaks dramatically increased as a result of the first cycle of voltammetric oxidation and reduction (compare the solid and dashed curves in Figure 5), also an indication of S desorption. Another clue as to the nature of the oxidation process is found from measurement of nox,the number of electrons required to oxidatively desorb an S atom: (4) nox = (Qox - Qb')/FArs where Q, is the charge to oxidize the surface and S layer (0.85 V), Q{ is the background charge to oxidize the clean Pt surface under the same conditions obtained from the positive-going scans in pure electrolyte (dotted line), F is the Faraday constant, A is the electrode area, and rs is the packng density of S. Positive-goingvoltammetric scans of S adsorbed at Pt(ll1) vary substantially with pH, Figure
5A-E. Perhaps most striking is the decrease in noxfrom about 6 below pH 11 to noxnear 3 at pH 14, Table I. Evidently, the oxidative desorption pathway of S from Pt(ll1) is influenced by pH. The fact that noxfor oxidation of Pt(lll)(d3Xd3)R30°-S equals about 6 at pH 9 is an indication that oxidation of adsorbed sulfur under these conditions proceeds to sulfate ion: S(ads) + 4H20 Sod2-+ 8H+ + 6e(5)
-
However, at higher pH the OH- ions promote desorption of sulfur in the forms of lower-valent species than S042or by direct replacement processes such as: S(ads) + OHOH(ads) + S + e(6) The EELS spectrum of the Pt(lll)(d3Xd3)R30°-S layer formed at -0.6 V at pH 9 is shown in Figure 6A. The principal feature of this spectrum is a shoulder at 325 cm-'
-
Langmuir, Vol. 5, No. 1, 1989 127
Structure and Composition of Pt Surfaces
ENERGY LOSS ( c m - I )
ENERGY L O S S ( c m - I )
0
1000 2000 3000 ENERGY LOSS (cm-I)
4000
100
IO00 2000 ENERGY L O S S ( E m - I )
3000
4000
Figure 6. EELS vibrational spectra. (A) Pt(111)(d3xd3)R30°-S. (B) As in A, after partial oxidative desorption of S (positive-going voltametric scan to 0.36 V in 10 mM NaF, pH 10). (C)As in A, after complete oxidative desorption of S (scan to to 0.720 V). (D) Clean Pt(ll1) surface after partial oxidation (positive-goingvoltammetric scan to 0.36 V in 10 mM NaF, pH 10). (E) As in D, after complete surface oxidation (scan to 0.720 V). Experimental conditions: EELS incidence and detection angles 62O from surface normal; beam energy, 4 eV; beam current, about 120 PA; EELS resolution, about 10 meV (80 cm-I) fwhm; other conditions as in Figure 4.
assignable to a Pt-S stretch. This shoulder has been observed previously for H2Sadsorption (225 K)at Pt(lll)." The EELS spectrum following the first oxidation peak at pH 10 (positive-going scan to 0.36 V, Figure 5C), is shown in Figure 6B. The conditions of this experiment resulted in removal of 87% of the initial S layer as determined by Auger spectroscopy. As would be expected, an EELS Pt-S stretch is barely detectable, Figure 6B. Instead, a prominent peak is present at 631 cm-', assignable to Pt-0 stretching, followed by three overlapping features at 990,1080, and 1200 cm-'. No vibrations were detectable near 3500 cm-l, indicating absence of surface 0-H. Accordingly, the vibrations near 1000 cm-' are not due primarily to 0-H bending modes, nor are they due primarily to sulfur oxygen compounds, (17) Koeetner, R.J.; Salmeron, M.;Kollin, E.G.; Gland, J. L.Surf. Sei. 1986, 172,668.
as can be seen from a comparison of Figure 6B with Figure 6D, obtained after scanning a clean Pt(ll1) surface to the same potential (0.36 Y). Similar behavior is observed at more positive potentials (0.72 V), Figure 6C and 6E. The remaining possibility is that the peaks at 990,1080, and 1200 cm-' are due to platinum complexes (Pt-02) of dioxygen species such as peroxide (O& and superoxide (02-). By comparison, the 0stretching frequency in the EELS spectrum of molecular dioxygen at Pt(ll1) (100 K) is 869 crn-'.l8 Presence of a sulfur layer a t the surface influences the final outcome of oxidation of the surface, as can be seen by comparing EELS spectra: Figure 6B with 6D or 6C with 6E. The principal effect of the sulfur layer is to increase the peak at 631 cm-l (Pt-0 stretch) while decreasing (or eliminating) the peak at 3559 cm-' (0-H stretch). That is, the presence of adsorbed sulfur (18) Sexton, B.A. Appl. Phys. 1981, A26, 1.
128 Langmuir, Vol. 5,No. 1, 1989
surface Pt(ll1)
pH 9
Table I. Auger and Electrochemical Data for Sulfur Adsorbed at Pt(ll1) and Pt(100) coulometric charge density, electrode potential, V Zs/Zptmo ZO/Z%O rs,nmol/cm2 ro,nmol/cm2 mC/cm2 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8
9.5 10.0
0.36 0.78
12.0 14.0 9
Pt(100)
Batina et al.
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8
9.5 10.0
0.61 0.87
7.68 7.24 7.60 7.24 2.62 1.04 0.187 0.159 1.00 0.0059 8.52 8.48 8.64 9.16 7.74 7.11 2.93 0.524 0.670 0.408
0.788 1.606 2.80 4.28
0.831 0.783 0.822 0.783 0.283 0.113 0.020 0.017
0.156 0.983 1.682 3.365
1.07 2.15
0.108 0.0006
0.843 1.69
0.165 0.694 2.01 2.83
1.08 1.08 1.10 1.07 0.902 0.372 0.067 0.064
0.166 0.648 2.022 2.85
2.20 2.44
0.085 0.052
2.21 2.46
12.0 14.0
increases the ratio of oxide to hydroxide species. Neglecting the dioxygen species for the moment, a plausible reaction equation is:
PtS + 5H20
-
PtO
+ S042- + 10H+ + 8e-
(7)
Oxidation of the Pt surface area vacated by S atoms accounts for two of the total eight electrons:
Pt
+ H20
-
PtO
+ 2H+ + 2e-
(8)
Oxidation of the Pt(ll1) surface beyond the main voltammetric peak (scanning more positive than 0.36 V) introduces 0-H bonds, such as would be found in Pt(OH)4 and related species: PtO
+ 3H20
-
Pt(OH),
+ 2H+ + 2e-
(9)
Extensive disordering of the surfaces as judged by LEED accompanies this latter stage of oxidation of the surface. Sulfide at Pt(lO0). Packing densities of sulfur and oxygen measured at Pt(100) over a range of electrode potential are given in Table I and Figure 4B. Desorption of S from Pt(100) at positive potentials is linked to adsorption of oxide species (02-, O : , and/or 027,Table I, similar to the behavior of Pt (111). LEED following immersion of Pt(100) into aqueous NazS solutions at pH 9 yields simple P t ( l l l ) ( d 2 x d 2 ) R45O-S patterns, as shown in Figure 2B. The structure corresponding to this pattern is shown in Figure 3B. The ideal packing density of S in this structure is 0s = 1/2 sulfur atom/surface Pt atom (rS= 1.08 nmol/cm2). Maximum sharpness of the LEED pattern occurs at -0.6 V. Accordingly, the S layer formed a t -0.6 V is taken as the packing density reference point. This leads to Bs = 7.88 cm2/nmol in eq 3. The Pt(lOO)(d2xd2)R45"-S structure is also formed when Pt(100) is treated with H2S in UHV at room temperat~re.'"'~ Packing densities of sulfur adsorbed at Pt(100) from aqueous solutions over a range of electrode potentials are given in Table I and Figure 4B. Positive-going voltammetric scans of S adsorbed at Pt(lOO),Figure 5F-J,are less sensitive to pH than those for Pt(ll1). However, noxdecreases with increasing pH
no1
533.8
6.7
466.1 281.5
5.8 3.5
291.5 296.1 598.6
3.6 3.7 5.8
591.4 465.7
5.7 4.5
513.8 413.5
4.9 4.0
as for Pt(ll1). That is, electrochemical oxidation of Pt(100)(~'2xd2)R45~-S proceeds to SO-: at pH near 9, as described by eq 5, with lesser contributions due to direct displacement of sulfur by oxygen (eq 6) at Pt(100) than at Pt(ll1).
Conclusions Adsorption of sulfur from aqueous sulfide solutions at Pt(ll1) and Pt(100) electrode surfaces is a function of electrode potential and crystal plane. Sulfur adsorption predominates at the Pt(100) surface at potentials less positive than 0.2 V (vs Ag/AgCl reference) and at the Pt(ll1) surface when the potential is less positive than 0.0 V (Figure 4). Absence of countercations from the sulfur-rich surface layer (Figure 1)is evidence that sulfur is present in the surface layer in an electroneutral form which can be visualized as adsorbed sulfur atoms or as sulfur atom ligands of the surface platinum atoms. At all potentials for which the sulfur layer is stable, the surface structure is primitive Pt(l00)(d2Xd2)R45O-S or Pt(111)(d3Xd3)R30°-S, respectively (1sulfur atom/unit mesh), Figures 2 and 3. The EELS spectrum of Pt(111)(v'3Xd3)R30°-S consists of a single Pt-S stretching vibration at 325 cm-', while partial oxidation of the layer leads to Pt-0 stretching a t 631 cm-' and vibrations attributable to dioxygen species at 990,1080, and 1200 cm-'. Stronger oxidation of the surface leads to an 0-H stretching bond a t 3559 cm-'. Voltammetric curves for oxidation of the adsorbed layers display distinctly different morphologies for the Pt(ll1) and Pt(100) surfaces and are pH-dependent. At pH 9-10, oxidation yields unadsorbed SO4" ions, while desorption involves less extensive oxidation of sulfur at more alkaline pH.
Acknowledgment. This work was supported by the US. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences. Instrumentation was provided by the National Science Foundation, the Air Force Office of Scientific Research, and the University of Cincinnati. Registry No. Pt, 7440-06-4; Na2S,1313-82-2; S2-, 18496-25-8; S, 7704-34-9; 02, 7782-44-7; S04'-, 14808-79-8.
