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Coadsorption of sulphur and hydrogen on Pt(111) studied by radiotracer and electrochemical techniques
Surface Science 169 (1986) L237-L244 North-Holland. Amsterdam
SURFACE
SCIENCE
L237
LETTERS
COADSORPTION OF SULPHUR AND HYDROGEN ON Pt( 111) STUDIED BY RADIOTRACER AND ELECTROCHEMICAL TECHNIQUES E. PROTOPOPOFF
and P. MARCUS
Lahoratorre de Physrco-Chmie da Surfaces, Assock au CNRS. C’A 425, Ecole Natwnale SupPrieure de Chink de Paris II, Rue Pierre et Marie Curre. F-75231 Paris Cedex 05. France Received
30 July 1985; accepted
for publication
2 December
1985
The influence of chemisorbed sulphur on the adsorption of hydrogen on Pt(ll1) in aqueous acid medium was quantitatively studied. Isotherms of electroadsorption of hydrogen on Pt(l1 I) covered with various amounts of sulphur are reported. The relation between the coverages of sulphur and hydrogen coadsorbed on the surface was established. The adsorption of hydrogen is totally inhibited by a sulphur coverage well below saturation coverage. The 0, versus 8, curve IS not linear. At low 0,. 8, decreases sharply with increasing 0s. The number of hydrogen adsorption sites blocked by one sulphur atoms is 8i 1. A model of nearest-neighbour sites poisoning fits well this experimental value. The same model of blocking also fits well the experimental variation up to Bs = 0.3, assuming a random distribution of sulphur. Above, the rapid fall-of of 8, to zero is best explained by a model of adsorption of hydrogen m an ordered sulphur overlayer structure. The shape of the isotherms indicates that sulphur weakens the Pt-H bond and reduces the interactions between H atoms.
In a previous paper [l], we have reported in detail the results of a quantitative study of the coadsorption of sulphur and hydrogen on Pt( 110) in an aqueous medium. We report here the results of a similar study performed on the (111) face. In contrast with the (110) face which is reconstructed in (1 lo)-(1 x 2), no reconstruction occurs on the (111) face. On this face several studies of electrochemical adsorption of hydrogen have been reported [2-111. However there is no general agreement on the saturation coverage and on the shape of the voltamograms. In this paper the effects of sulphur on the isotherms of electroadsorption of hydrogen on Pt( 111) are reported and the relation between the hydrogen coverage (19,) and the sulphur coverage (8,) is given. From these results information on the interactions between H and S adsorbed atoms can be derived. The preparation of the samples and the techniques used were described previously [l]. Sulphur was preadsorbed in a H,S/H, gaseous mixture and the 0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
1.238
E. Protopopoff P. Marcus / Sulphur and t~vdrogen on Pt(l 11)
sulphur coverage was measured by' a radiochemical method using the )5S isotope [12]. Hydrogen was adsorbed from an acid aqueous electrolyte (0.05M H2SO4) and its coverage measured using cyclic voltametry. Sulphur was adsorbed on the P t ( l l l ) surface in H 2 S / H 2 under th,~, following conditions: p(H2S)/p(H2)= 10 3, it'= 350oC. Under these conditions, the saturation coverage with sulphur is obtained. The amount of sulphur is 39 + 2 ng cm 2 a value which corresponds to a ratio S / P t of ~ 0.5. This is in agreement with earlier results [13]. After preparation of the monolayer, the crystal was transferred into the glove box where the electrochemical measurements were performed. It was checked that sulphur is stable in ,Ire region of adsorption/desorption of hydrogen. All sulphur coverages below q,ne complete monolayer were produced from saturation coverage by' electro-o, lation, in a way described previously [1]. The potentials used there were between 1.1 and 1.5 V, depending on the desired oxidation kinetics. Fig. 1 shows a series of hydrogen desorption peaks for various sulphur coverages between 0 and 0.5. As for the (110) face, it is observed that the adsorption of hydrogen is drastically influenced by preadsorbed sulphur. The adsorption of hydrogen is totally inhibited on surfaces with 8s > 0.36. The hydrogen coverage H / P t on the sulphur free surface is 0.97 _+ 0.05 (235 /,tC cm 2). The removal of sulphtJr requires several oxidation/reduction cycles up to 1.5 V. This was explained previously [1]. Numerous cycles in the oxide formation region may disturb the surface, as evidenced by LEED spot broadening [11]. However, this treatment does not seem to change the value of the maximum H coverage [9] and a value of 240 ~C cm 2 was measured on Pt(111) not cycled in the oxide formation region [7]. Fig. 2a shows the isotherms of hydrogen adsorption on surfaces covered with various amounts of sulphur. The strong inhibiting effect of sulphur on the adsorption of hydrogen is observed at all 0s and at all hydrogen equivalent pressures. Before discussing further the main effect which is the blocking by sulphur, we first consider the influence of sulphur on the adsorption of hydrogen on the sites not blocked by sulphur. The isotherms in fig. 2b are normalized with respect to the maximum 0 u obtained for each value of 0s. It is observed that sulphur changes the shape of the isotherms. The following effects of sulphur are observed: (i) The isotherms are shifted to higher pressures when the sulphur coverage increases. Assuming that the hydrogen adsorption entropy is not changed by sulphur, it shows that the H Pt bond energy is lowered in the presence of adsorbed sulphur. A decrease of the enthalpy of = 8 kJ m o l J (85 + 10 mV) is measured at OH ---, 0 when 0s increases from 0 to 0.33 (26 ng cm 2). At a hydrogen coverage at half-saturation, the variation is only = 2 kJ tool i (20 mV). (ii) The isotherms are steeper when the surface is covered by sulphur. In fla. 2b is shown a Langmuir isotherm, calculated with a
E. Protopopoff P. Marcus / Sulphur and hydrogen on Pt(l l l)
L239
b
E
(J ::L
H,S/Pt
(111)
~3C gl e-
"o
20
0.07 0.16
10
0.22
0 ~ 0
0.1
0.2
0.3 0.4 p o t e n t i a l , V/RHE
Fig. 1. Hydrogen adsorption voltamograms on P t ( l l l ) for various sulphur coverages (0.05M H2SO4,20 mV s l).
standard free energy of hydrogen adsorption derived from the potential at half-saturation coverage in the experimental curve for the clean surface. The deviation of the isotherm .of the clean surface from the ideal isotherm indicates repulsive interactions between adsorbed H atoms. Our results show that sulphur lowers these interactions. This probably originates from the dilution of hydrogen by sulphur. At 0s = 0.33 the isotherm is very similar to the Langmuir isotherm. It may be noted that the isotherm for 0s = 0.07 is slightly shifted to lower pressures compared to the isotherm for the clean surface. However this shift is almost within the experimental error ( _+5 mV) and therefore this minor effect will not be discussed here. We now consider quantitatively the blocking effect of sulphur on the adsorption of hydrogen. The OH versus 0s curve in fig. 3 has been obtained from the isotherms of fig. 2a, taking the maximum hydrogen coverage at 0 V for each sulphur coverage. The OH versus 0s curve is not linear and a rapid decrease of 0 n with increasing 0s is obtained. The adsorption of hydrogen is totally inhibited for 0s ~ 0.36 (29 ng cm-2), i.e. well below the sulphur
L240
E. Protopopof]] P. Marcus / Sulphur and hydrogen on Pt( l 11)
I"01 a
H,S
Pt (111)
~
; " --]
/ / ' 0 s, 0
o.8l /i / /'
o06~ // /' /
z/~
0.2
oS
.......
0.3 potential, V RHE
e s o 0.07
0.2
t
0.1
0
In PH2, Pa -10
-8
-6
-4
-2
0
2
log PH2,Tor r 1.0
...........
._~>....-
H,S Pt (111)
lB
/""
//
g O
,3o.6 ..r
~0.4 = 0.16
/ ~19s:0.22
0.2
/ ......
0.4
0.3 potential, V RHE -15
-10
-10 -8
-6
. ~s:
0.2 -5 -4
0.33
0.1 0 -2
0 5 10 In PH2, Pa 0
2
log PH2,Torr Fig. 2. (a) Isotherms of hydrogen electroad'sorption on Pt(ll 1) for ;,arious sulphur coverages. (b) Normalized isotherms of hydrogen electroadsorption on Pt(l I 1) for various sulphur coverages. The dotted curve is a calculated Langmuir isotherm.
E. Protopopoff P. Marcus / Sulphur and hydrogen on Pt(l l l)
I"C~I~,
L241
H,SPt(111)
go.8 ~', 0 U
~ \
gO.
~
~
', \,
"r 0.2
0
0
0.1
0.2
0.3 0.4 0.5 Sulphur coverage 0 s Fig. 3. Hydrogen saturation coverage versus the sulphur coverage. The dotted curve is calculated according to OH = (1 0s) 7.
saturation coverage. The slope of the 0 H versus 0 s curve at low 0s is found to be 8 _+ 1. This value gives the number of hydrogen adsorption sites blocked by one sulphur atom isolated on the surface. It can be explained by a simple nearest-neighbour model, where a single S atom blocks all H adsorption sites involving one Pt atom in nearest-neighbour position to the S atom. Regarding the adsorption sites, it has been shown by He diffraction experiments on a H / P t (111) surface [14] that H atoms are adsorbed in one type of 3-fold hollow sites and it is likely that S atoms should be adsorbed in sites of the same coordination. Then values of 7 or 6 sites blocked per sulphur atom are found (fig. 4), depending whether H is adsorbed on the same 3-fold sites as sulphur, or on different 3-fold sites. (Indeed there are two kinds of 3-fotd hollow sites on P t ( l l l ) , those that overlie the 2nd layer atoms, and those that overlie the hollows of the 2nd layer). In both cases, three Pt atoms are poisoned by one S atom. The theoretical value of 7 is in good agreement with the experimental value of 8 _+ 1. At higher sulphur coverages, the decrease of OH is less marked. This is expected because, when increasing 0s, the zones of influence of S atoms will overlap. Assuming that S is randomly distributed, the H coverage is equal to the probability of finding 7 sulphur-free sites. This may be expressed by OH = (1 - 0s) 7. This relation is shown in fig. 3 with the experimental 0 H versus 0 s curve. The fit is quite good, up to 0 s ~ 0.3. Above, the relation does not fit the experimental curve. Previous L E E D studies of the sulphur adsorption from
L242
E. Protopopof/] P. Marcus / Sulphur and hydrogen on Pttl I 1)
(a)
(b)
Fig. 4. S d l c m a t i c r e p r e s e n t a t i o n of the /x)ne u l f l u c n c c d b'~ an i~olatcd ~ulphur a t o m on a (l 1 l) surfacc. T w o possible cases are shox~n: (a) H is a d s o r b e d on the s a m e 3-fold sitc~,as s u l p h u r : (b) H is a d s o r b e d on the o t h e r 3-fold silos.
a gas phase [13] have shown that no p a t t e r n is observed for 0 s < 0.05. At increasing S coverage, a p(2 x 2) and then a v/3 × v"3 R30 ° overlayer structure were observed. As shown above, our results suggest that in acid aqueous m e d i u m , sulphur is r a n d o m l y d i s t r i b u t e d up to 0 s ~ 0.3. The ~/_~ x ~/3 R30 ° structure was shown to be stable in the range 0s-= 0.25 (20 ng cm :* to 0s = 0 . 3 8 ( 3 0 n g c m 2 ) [13]. This structure is s t o i c h i o m e t r i c at 0s = 0 . 3 3 . If this structure is assumed to exist in the electrolyte, the n e a r e s t - n e i g h b o u r b l o c k i n g effect described before predicts thai 0 u = 0 for 0 s = 0.33. This fits the experimental restllts. The main effects of sulphur on the a d s o r p t i o n of h y d r o g e n on P t ( l l l ) are the following: the a d s o r p t i o n of h y d r o g e n is totally p r e c l u d e d for a sulphur coverage (0 s = 0.36) well below one c o m p l e t e m o n o l a y e r (0.5); the 0 u 0s curve is not linear: for low 0s, 0it decreases sharply: the n u m b e r of h y d r o g e n a d s o r p t i o n sites b l o c k e d by one isolated S atom. given by the slope at the origin, is 8 ± 1, i n d i c a t i n g that all H a d s o r p t i o n sites involving at least one Pt a t o m in n e a r e s t - n e i g h b o u r position to a S a t o m are d e a c t i v a t e d : when 0s increases, the same m o d e l of blocking is valid up to Üs = 0.3, a s s u m i n g a r a n d o m d i s t r i b u t i o n of sulphur (statistical model); the same n e a r e s t - n e i g h b o u r b l o c k i n g effect explains 0u = 0 for 0s >~ 0.36 if the a s s u m p t i o n is m a d e that s u l p h u r is a d s o r b e d in an o r d e r e d ~ x ,/3 R30 ° structure similar to that o b s e r v e d in the gas p h a s e [13] (structural model), c o n c e r n i n g the influence of s u l p h u r on the H a d s o r p t i o n sites which are not blocked, it has been shown that sulphur weakens the P t - H b o n d by = 8 kJ mol i at #it --+ 0, but lowers the interactions between a d s o r b e d H atoms, p r o b a b l y by a dilution effect; this c o m p e n s a t i o n results in a smaller effect of sulphur on the Pt--H b o n d at higher C o m p a r i s o n of these results to those o b t a i n e d previously on P t ( l l 0 ) shows that the effects of sulphur on H a d s o r p t i o n are essentially the same the two faces. However, some points are characteristic of the crystal face: statistical model does not a p p l y to P t ( l l 0 ) , while it fits rather well
[1] on the the
E. Protopopoff, P. Marcus / S u l p h u r and hydrogen on Pt(l l l)
L243
experimental variation on P t ( l l l ) , up to 0s-~0.3; on the (110) face the structural model of adsorption of H in the structures formed by sulphur [15] fits the experimental variation for 0s > 0.3, but only on the basis of a blocking effect more localized than at low 0s [1]; on the (111) face the n e a r e s t - n e i g h b o u r blocking effect of sulphur adsorbed in the (3- × ~f3 R30 ° structure explains well the rapid fall-off of OH to zero for 0s > 0.3: the weakening effect of s u l p h u r on the P t - H b o n d at low 0s is stronger on the (111) face than on the (110) face; on the (110) face the lowering of the interaction between H atoms by sulphur results in a stabilization of the Pt H b o n d at high 0 H, while on the (111) face, due to the stronger effect of sulphur on the Pt - H bond, there is no stabilization at high 0 H. Besides there m i n o r differences of behaviour of the two faces, the main effects of sulphur on hydrogen adsorption, emerging from these studies of P t ( l l 0 ) [1] and P t ( l l ) are the following: - The surface is totally poisoned for adsorption of hydrogen when it is saturated with sulphur. - Sulphur is more stable than hydrogen, and no desorption of sulphur is caused by a d s o r p t i o n of hydrogen. - The OH versus 0s curve is not linear and is very steep at low 0 s, indicating that an isolated sulphur atom blocks all the hydrogen adsorption sites involving at least one p l a t i n u m atom in n e a r e s t - n e i g h b o u r position to the S atom. The variation of the hydrogen coverage for 0s > 0.3 can be explained by models considering that the sulphur overlayer structures observed in the gas phase [13,15] are the same in the aqueous phase. Sulphur weakens the P t - H b o n d at 0 H ~ 0, but lowers the interactions between adsorbed H atoms at increasing hydrogen coverage. The authors are grateful to Professor J. O u d a r for fruitful discussions d u r i n g the course of this work. F i n a n c i a l support from the D G R S T u n d e r contract No. 81 F 1032 is acknowledged.
References
[1] [2] [3] [4] [5] [6] [7]
P. Marcus and E. Protopopoff, Surface Sci. 161 (1985) 533. F.G. Will, J. Electrochem. Soc. 112 (1965) 451. A. Hubbard, R, Ishikawa and J. Katekaru, J. Electoanal. Chem. 86 (1978) 271. E. Yeager, W.E. O'Grady, M. Woo and P. Hagans, J. Electrochem. Soc. 125 (1978) 348. P.N. Ross, Jr., J. Electrochem. Soc. 126 (1979) 67. K. Yamamoto, D.M. Kolb, R. K6tz and G. Lehmpfuhl, J. Electroanal. Chem. 96 (1979) 233. J. Clavilier, R. Faure, G. Guinet and R. Durand, J. Electroanal. Chem. 107 (1980) 205: J. Clavilier, J. Electroanal. Chem. 107 (1980) 211. [8] P.N. Ross, Jr., Surface Sci. 102 (1981) 463. [9] C.L. Scortichini and C.N. Reilley, J. Electroanal. Chem. 139 (1982) 247. [10] A.S. Homa, E. Yeager and B.D. Cahan, J. Electroanal. Chem. 150 (1983) 181.
L244 [11] [12] [13] [14] [15]
E. Protopopoff P. Marcus / S u l p h u r and hydrogen on Pt(l l l ) F.T. Wagner and P.N. Ross, Jr., J. IElectroanal. Chem. 150 (1983) 141. J. Oudar, Compt. Rend. (Paris) 249 (1959) 91. Y. Berthier, M. Perdereau and J. Oudar, Surface Sci. 36 (1973) 225. J. Lee, J.P. Cowin and L. Warthon, Surface Sci. 130 (1983) 1. Y. Berthier, J. Oudar and M. Huber, Surface Sci. 65 (1977) 361.
SURFACE
SCIENCE
L237
LETTERS
COADSORPTION OF SULPHUR AND HYDROGEN ON Pt( 111) STUDIED BY RADIOTRACER AND ELECTROCHEMICAL TECHNIQUES E. PROTOPOPOFF
and P. MARCUS
Lahoratorre de Physrco-Chmie da Surfaces, Assock au CNRS. C’A 425, Ecole Natwnale SupPrieure de Chink de Paris II, Rue Pierre et Marie Curre. F-75231 Paris Cedex 05. France Received
30 July 1985; accepted
for publication
2 December
1985
The influence of chemisorbed sulphur on the adsorption of hydrogen on Pt(ll1) in aqueous acid medium was quantitatively studied. Isotherms of electroadsorption of hydrogen on Pt(l1 I) covered with various amounts of sulphur are reported. The relation between the coverages of sulphur and hydrogen coadsorbed on the surface was established. The adsorption of hydrogen is totally inhibited by a sulphur coverage well below saturation coverage. The 0, versus 8, curve IS not linear. At low 0,. 8, decreases sharply with increasing 0s. The number of hydrogen adsorption sites blocked by one sulphur atoms is 8i 1. A model of nearest-neighbour sites poisoning fits well this experimental value. The same model of blocking also fits well the experimental variation up to Bs = 0.3, assuming a random distribution of sulphur. Above, the rapid fall-of of 8, to zero is best explained by a model of adsorption of hydrogen m an ordered sulphur overlayer structure. The shape of the isotherms indicates that sulphur weakens the Pt-H bond and reduces the interactions between H atoms.
In a previous paper [l], we have reported in detail the results of a quantitative study of the coadsorption of sulphur and hydrogen on Pt( 110) in an aqueous medium. We report here the results of a similar study performed on the (111) face. In contrast with the (110) face which is reconstructed in (1 lo)-(1 x 2), no reconstruction occurs on the (111) face. On this face several studies of electrochemical adsorption of hydrogen have been reported [2-111. However there is no general agreement on the saturation coverage and on the shape of the voltamograms. In this paper the effects of sulphur on the isotherms of electroadsorption of hydrogen on Pt( 111) are reported and the relation between the hydrogen coverage (19,) and the sulphur coverage (8,) is given. From these results information on the interactions between H and S adsorbed atoms can be derived. The preparation of the samples and the techniques used were described previously [l]. Sulphur was preadsorbed in a H,S/H, gaseous mixture and the 0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
1.238
E. Protopopoff P. Marcus / Sulphur and t~vdrogen on Pt(l 11)
sulphur coverage was measured by' a radiochemical method using the )5S isotope [12]. Hydrogen was adsorbed from an acid aqueous electrolyte (0.05M H2SO4) and its coverage measured using cyclic voltametry. Sulphur was adsorbed on the P t ( l l l ) surface in H 2 S / H 2 under th,~, following conditions: p(H2S)/p(H2)= 10 3, it'= 350oC. Under these conditions, the saturation coverage with sulphur is obtained. The amount of sulphur is 39 + 2 ng cm 2 a value which corresponds to a ratio S / P t of ~ 0.5. This is in agreement with earlier results [13]. After preparation of the monolayer, the crystal was transferred into the glove box where the electrochemical measurements were performed. It was checked that sulphur is stable in ,Ire region of adsorption/desorption of hydrogen. All sulphur coverages below q,ne complete monolayer were produced from saturation coverage by' electro-o, lation, in a way described previously [1]. The potentials used there were between 1.1 and 1.5 V, depending on the desired oxidation kinetics. Fig. 1 shows a series of hydrogen desorption peaks for various sulphur coverages between 0 and 0.5. As for the (110) face, it is observed that the adsorption of hydrogen is drastically influenced by preadsorbed sulphur. The adsorption of hydrogen is totally inhibited on surfaces with 8s > 0.36. The hydrogen coverage H / P t on the sulphur free surface is 0.97 _+ 0.05 (235 /,tC cm 2). The removal of sulphtJr requires several oxidation/reduction cycles up to 1.5 V. This was explained previously [1]. Numerous cycles in the oxide formation region may disturb the surface, as evidenced by LEED spot broadening [11]. However, this treatment does not seem to change the value of the maximum H coverage [9] and a value of 240 ~C cm 2 was measured on Pt(111) not cycled in the oxide formation region [7]. Fig. 2a shows the isotherms of hydrogen adsorption on surfaces covered with various amounts of sulphur. The strong inhibiting effect of sulphur on the adsorption of hydrogen is observed at all 0s and at all hydrogen equivalent pressures. Before discussing further the main effect which is the blocking by sulphur, we first consider the influence of sulphur on the adsorption of hydrogen on the sites not blocked by sulphur. The isotherms in fig. 2b are normalized with respect to the maximum 0 u obtained for each value of 0s. It is observed that sulphur changes the shape of the isotherms. The following effects of sulphur are observed: (i) The isotherms are shifted to higher pressures when the sulphur coverage increases. Assuming that the hydrogen adsorption entropy is not changed by sulphur, it shows that the H Pt bond energy is lowered in the presence of adsorbed sulphur. A decrease of the enthalpy of = 8 kJ m o l J (85 + 10 mV) is measured at OH ---, 0 when 0s increases from 0 to 0.33 (26 ng cm 2). At a hydrogen coverage at half-saturation, the variation is only = 2 kJ tool i (20 mV). (ii) The isotherms are steeper when the surface is covered by sulphur. In fla. 2b is shown a Langmuir isotherm, calculated with a
E. Protopopoff P. Marcus / Sulphur and hydrogen on Pt(l l l)
L239
b
E
(J ::L
H,S/Pt
(111)
~3C gl e-
"o
20
0.07 0.16
10
0.22
0 ~ 0
0.1
0.2
0.3 0.4 p o t e n t i a l , V/RHE
Fig. 1. Hydrogen adsorption voltamograms on P t ( l l l ) for various sulphur coverages (0.05M H2SO4,20 mV s l).
standard free energy of hydrogen adsorption derived from the potential at half-saturation coverage in the experimental curve for the clean surface. The deviation of the isotherm .of the clean surface from the ideal isotherm indicates repulsive interactions between adsorbed H atoms. Our results show that sulphur lowers these interactions. This probably originates from the dilution of hydrogen by sulphur. At 0s = 0.33 the isotherm is very similar to the Langmuir isotherm. It may be noted that the isotherm for 0s = 0.07 is slightly shifted to lower pressures compared to the isotherm for the clean surface. However this shift is almost within the experimental error ( _+5 mV) and therefore this minor effect will not be discussed here. We now consider quantitatively the blocking effect of sulphur on the adsorption of hydrogen. The OH versus 0s curve in fig. 3 has been obtained from the isotherms of fig. 2a, taking the maximum hydrogen coverage at 0 V for each sulphur coverage. The OH versus 0s curve is not linear and a rapid decrease of 0 n with increasing 0s is obtained. The adsorption of hydrogen is totally inhibited for 0s ~ 0.36 (29 ng cm-2), i.e. well below the sulphur
L240
E. Protopopof]] P. Marcus / Sulphur and hydrogen on Pt( l 11)
I"01 a
H,S
Pt (111)
~
; " --]
/ / ' 0 s, 0
o.8l /i / /'
o06~ // /' /
z/~
0.2
oS
.......
0.3 potential, V RHE
e s o 0.07
0.2
t
0.1
0
In PH2, Pa -10
-8
-6
-4
-2
0
2
log PH2,Tor r 1.0
...........
._~>....-
H,S Pt (111)
lB
/""
//
g O
,3o.6 ..r
~0.4 = 0.16
/ ~19s:0.22
0.2
/ ......
0.4
0.3 potential, V RHE -15
-10
-10 -8
-6
. ~s:
0.2 -5 -4
0.33
0.1 0 -2
0 5 10 In PH2, Pa 0
2
log PH2,Torr Fig. 2. (a) Isotherms of hydrogen electroad'sorption on Pt(ll 1) for ;,arious sulphur coverages. (b) Normalized isotherms of hydrogen electroadsorption on Pt(l I 1) for various sulphur coverages. The dotted curve is a calculated Langmuir isotherm.
E. Protopopoff P. Marcus / Sulphur and hydrogen on Pt(l l l)
I"C~I~,
L241
H,SPt(111)
go.8 ~', 0 U
~ \
gO.
~
~
', \,
"r 0.2
0
0
0.1
0.2
0.3 0.4 0.5 Sulphur coverage 0 s Fig. 3. Hydrogen saturation coverage versus the sulphur coverage. The dotted curve is calculated according to OH = (1 0s) 7.
saturation coverage. The slope of the 0 H versus 0 s curve at low 0s is found to be 8 _+ 1. This value gives the number of hydrogen adsorption sites blocked by one sulphur atom isolated on the surface. It can be explained by a simple nearest-neighbour model, where a single S atom blocks all H adsorption sites involving one Pt atom in nearest-neighbour position to the S atom. Regarding the adsorption sites, it has been shown by He diffraction experiments on a H / P t (111) surface [14] that H atoms are adsorbed in one type of 3-fold hollow sites and it is likely that S atoms should be adsorbed in sites of the same coordination. Then values of 7 or 6 sites blocked per sulphur atom are found (fig. 4), depending whether H is adsorbed on the same 3-fold sites as sulphur, or on different 3-fold sites. (Indeed there are two kinds of 3-fotd hollow sites on P t ( l l l ) , those that overlie the 2nd layer atoms, and those that overlie the hollows of the 2nd layer). In both cases, three Pt atoms are poisoned by one S atom. The theoretical value of 7 is in good agreement with the experimental value of 8 _+ 1. At higher sulphur coverages, the decrease of OH is less marked. This is expected because, when increasing 0s, the zones of influence of S atoms will overlap. Assuming that S is randomly distributed, the H coverage is equal to the probability of finding 7 sulphur-free sites. This may be expressed by OH = (1 - 0s) 7. This relation is shown in fig. 3 with the experimental 0 H versus 0 s curve. The fit is quite good, up to 0 s ~ 0.3. Above, the relation does not fit the experimental curve. Previous L E E D studies of the sulphur adsorption from
L242
E. Protopopof/] P. Marcus / Sulphur and hydrogen on Pttl I 1)
(a)
(b)
Fig. 4. S d l c m a t i c r e p r e s e n t a t i o n of the /x)ne u l f l u c n c c d b'~ an i~olatcd ~ulphur a t o m on a (l 1 l) surfacc. T w o possible cases are shox~n: (a) H is a d s o r b e d on the s a m e 3-fold sitc~,as s u l p h u r : (b) H is a d s o r b e d on the o t h e r 3-fold silos.
a gas phase [13] have shown that no p a t t e r n is observed for 0 s < 0.05. At increasing S coverage, a p(2 x 2) and then a v/3 × v"3 R30 ° overlayer structure were observed. As shown above, our results suggest that in acid aqueous m e d i u m , sulphur is r a n d o m l y d i s t r i b u t e d up to 0 s ~ 0.3. The ~/_~ x ~/3 R30 ° structure was shown to be stable in the range 0s-= 0.25 (20 ng cm :* to 0s = 0 . 3 8 ( 3 0 n g c m 2 ) [13]. This structure is s t o i c h i o m e t r i c at 0s = 0 . 3 3 . If this structure is assumed to exist in the electrolyte, the n e a r e s t - n e i g h b o u r b l o c k i n g effect described before predicts thai 0 u = 0 for 0 s = 0.33. This fits the experimental restllts. The main effects of sulphur on the a d s o r p t i o n of h y d r o g e n on P t ( l l l ) are the following: the a d s o r p t i o n of h y d r o g e n is totally p r e c l u d e d for a sulphur coverage (0 s = 0.36) well below one c o m p l e t e m o n o l a y e r (0.5); the 0 u 0s curve is not linear: for low 0s, 0it decreases sharply: the n u m b e r of h y d r o g e n a d s o r p t i o n sites b l o c k e d by one isolated S atom. given by the slope at the origin, is 8 ± 1, i n d i c a t i n g that all H a d s o r p t i o n sites involving at least one Pt a t o m in n e a r e s t - n e i g h b o u r position to a S a t o m are d e a c t i v a t e d : when 0s increases, the same m o d e l of blocking is valid up to Üs = 0.3, a s s u m i n g a r a n d o m d i s t r i b u t i o n of sulphur (statistical model); the same n e a r e s t - n e i g h b o u r b l o c k i n g effect explains 0u = 0 for 0s >~ 0.36 if the a s s u m p t i o n is m a d e that s u l p h u r is a d s o r b e d in an o r d e r e d ~ x ,/3 R30 ° structure similar to that o b s e r v e d in the gas p h a s e [13] (structural model), c o n c e r n i n g the influence of s u l p h u r on the H a d s o r p t i o n sites which are not blocked, it has been shown that sulphur weakens the P t - H b o n d by = 8 kJ mol i at #it --+ 0, but lowers the interactions between a d s o r b e d H atoms, p r o b a b l y by a dilution effect; this c o m p e n s a t i o n results in a smaller effect of sulphur on the Pt--H b o n d at higher C o m p a r i s o n of these results to those o b t a i n e d previously on P t ( l l 0 ) shows that the effects of sulphur on H a d s o r p t i o n are essentially the same the two faces. However, some points are characteristic of the crystal face: statistical model does not a p p l y to P t ( l l 0 ) , while it fits rather well
[1] on the the
E. Protopopoff, P. Marcus / S u l p h u r and hydrogen on Pt(l l l)
L243
experimental variation on P t ( l l l ) , up to 0s-~0.3; on the (110) face the structural model of adsorption of H in the structures formed by sulphur [15] fits the experimental variation for 0s > 0.3, but only on the basis of a blocking effect more localized than at low 0s [1]; on the (111) face the n e a r e s t - n e i g h b o u r blocking effect of sulphur adsorbed in the (3- × ~f3 R30 ° structure explains well the rapid fall-off of OH to zero for 0s > 0.3: the weakening effect of s u l p h u r on the P t - H b o n d at low 0s is stronger on the (111) face than on the (110) face; on the (110) face the lowering of the interaction between H atoms by sulphur results in a stabilization of the Pt H b o n d at high 0 H, while on the (111) face, due to the stronger effect of sulphur on the Pt - H bond, there is no stabilization at high 0 H. Besides there m i n o r differences of behaviour of the two faces, the main effects of sulphur on hydrogen adsorption, emerging from these studies of P t ( l l 0 ) [1] and P t ( l l ) are the following: - The surface is totally poisoned for adsorption of hydrogen when it is saturated with sulphur. - Sulphur is more stable than hydrogen, and no desorption of sulphur is caused by a d s o r p t i o n of hydrogen. - The OH versus 0s curve is not linear and is very steep at low 0 s, indicating that an isolated sulphur atom blocks all the hydrogen adsorption sites involving at least one p l a t i n u m atom in n e a r e s t - n e i g h b o u r position to the S atom. The variation of the hydrogen coverage for 0s > 0.3 can be explained by models considering that the sulphur overlayer structures observed in the gas phase [13,15] are the same in the aqueous phase. Sulphur weakens the P t - H b o n d at 0 H ~ 0, but lowers the interactions between adsorbed H atoms at increasing hydrogen coverage. The authors are grateful to Professor J. O u d a r for fruitful discussions d u r i n g the course of this work. F i n a n c i a l support from the D G R S T u n d e r contract No. 81 F 1032 is acknowledged.
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