J. Phys. Chem. 1993,97, 47694116
4769
Hydrogen Evolution on Pt Single Crystal Surfaces. Effects of Irreversibly Adsorbed Bismuth and Antimony on Hydrogen Adsorption and Evolution on Pt( 100) R. C6mez, A. FemBndez-Vega, J. M. Feliu,’ and A. Aldaz Departament de Quimica Fisica, Universitat d’Alacant, Apartat 99, 03080 Alacant. Spain Received: January 5, 1993
A study of the hydrogen evolution reaction (HER) has been carried out on platinum (1 1l), (loo), and (1 10) oriented electrodes in acidic medium (H2S04 0.5 M). No influence of the surface structure of the electrode has been observed. It has been confirmed that Hufl cannot be the intermediate involved in the hydrogen evolution reaction. A detailed investigation follows concerning the HER on Pt( 100) electrodes poisoned by irreversibly adsorbed adatoms. The Bi-Pt(100) and Sb-Pt(l00) systems have been studied and in both cases the same general behavior has been observed. The basic HER mechanism does not change with adatom adsorption, while its rate is lowered as a consequence of poisoning. The simultaneous investigation of the HER and the underpotentially deposited hydrogen adsorption in such systems can be used to obtain information about the compactness and arrangement of the adlayers. From this investigation, a negligible electronic effect of these adatoms on HER has been pointed out.
Introduction The hydrogen evolution reaction (HER) has been one of the most intensively investigated electrode processes in electrochemistry due to its special historical place. A general mechanism has been proposed,lv2 which consists in an initial discharge step
followed by either the chemical recombination step or the electrochemical desorption step
+ + e-+
Hads H+
H2
(3)
The mechanism commonly accepted on platinum at low overpotentials is a discharge step in equilibrium followed by a chemical recombination ~ t e p . ~ , ~ However, some details of the mechanism are not fully understood. Among them we find the problem of the nature of the adsorbed intermediate involved in the evolution reaction (Hu@ or H*) and its mode of bonding on the surface. Recently, it has been shown that the HER is insensitive to the surface structure of platinum in spite of the great variation of the standard Gibbs adsorption energy of Hu@with the different crystallographic orientations.s.6 Although several platinum orientations behave similarly with respect to the HER, the reported experimentsshow the presence of uncontrolled contaminants on the electrode surfaces that can modify the surface recombination step ( 2 ) of the mechanism.3 Also, the voltammetricprofiles show significant contributions of randomly distributed defects as a consequence of the surface pretreatment5 or preparation6 In this paper, the HER has been investigated on the three basal planes of platinum containing a low amount of surface defects and controlling the surface cleanliness in order to ensure the character of structure insensitivity of this reaction. In order to put in agreement the proposed mechanism for platinum electrodes and the experimental values of the Tafel slopes, very low coverages of hydrogen are required at the onset of the evolution reaction.] However, it is well-known that UPD hydrogen covers the electrode surface to a great extent at the HER potential range, thus making doubtful the role of these hydrogen adatoms on the HER. Working with polyoriented Pt Send correspondence to this author.
0022-3654 f 93/2091-4169S04.00/0
electrodes subjected to different pretreatments, Schuldiner’ proposed that there were two different types of hydrogen adatoms on the surface. It would be interesting to find new evidence for this suggestion. On the other hand, the HER is known to be very sensitive to the presence of trace contaminant^.^ Small amounts of poison can drastically reduce thereaction rate. In the past, the inhibition of the HER by various foreign adatomsunderpotentially deposited in a conventional way has been studied.8 However, these investigations could not allow a detailed elucidation of the poisoning mechanism that can be characterized only if the coverage of poisoning species on the surface remains constant in the whole potential range studied and if single-crystal electrodes are used instead of classical Pt polycrystals. This is because the poison should be adsorbed at a different rate on the different surface sites and also their inhibition behavior could be different. The hydrogen reaction has also been studied with control of the surface composition and structure in presence of irreversibly adsorbed s u l f ~ r . ~InJ the ~ same manner, it has been shown that arseniccan be irreversibly adsorbed on a Pt( 1 1 1 ) electrodesurface in a controlled way and its influence on the behavior of the hydrogen evolution reaction has been investigated.” The latter work shows that hydrogen adsorption can be completely blocked without significant modification of the HER mechanism. The aim of this work is to study the influence of irreversibly adsorbed adatoms of Bi and Sb on the hydrogen adsorption and on the hydrogen evolution reaction on Pt(100). The results give some insight on the arrangement of the metallic adatom adlayer responsible of the inhibition effect.
Experimental Section Cell, electrodes, apparatus, and techniques have been described elsewhere.I 2 ~ 1 3 The test electrolyte was 0.5 M HzSO4 used to check the surface redox reaction of bismuth and antimony adspecies as well as the hydrogen adsorption on the remaining free platinum sites. The hydrogen evolution reaction was also studiedby using this solution. Bismuth- and antimony-modified Pt( 100) electrodes were prepared by immersion of the clean electrodes in IW-10-5 M Biz03 (Merck pa.) or Sb2O3 (Merck p.a.) dissolved in 0.5 M sulfuric acid solution. Different coverages of these adatoms have been obtained either directly or after partial desorption of more covered initial layers.l2-I4 0 1993 American Chemical Society
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The Journal of Physical Chemistry, Vol, 97, No. 18, 1993
Gbmez et al.
After characterizing the adelectrode, the i vs E curves for the HER were immediately recorded by sweeping negatively the potential at a sweep rate of 5 mV/s. The current densities were calculated from the measured current on the basis of the geometric area of the electrode. The adatom coverage is calculated as
QAd/nQHP' (4) where Q A corresponds ~ to the charge exchanged during the adatom, surface redox process, n corresponds to the number of electrons exchanged during the same process, and Q H ~represents ' the H,,@ adsorption charge for the bare surface. It must be pointed out that this equation implies that coverages will be expressed as the number of adatoms per surface platinum site. All the experiments were made at room temperature and all potentials measured against the RHE electrode.
Results
Bare Platinum. Experiments have been performed with electrodes having the three basal orientations: Pt(l1 l), Pt( loo), and Pt( 110). Therepresentativevoltammogramsin 0.5 M sulfuric acid for the initial state of each electrode before each experiment are shown in Figure 1. The process used to characterize the electrodes is the adsorption-desorption of hydrogen at underpotential, Le., HU@. The voltammetric profiles obtained for each electrode show the characteristic features assigned to well-ordered surfaces after flame treatment.I4-l6 A very small amount of defects can be detected. These voltammograms remain stable upon successive cycles in the potential range shown in the figure, thus suggesting the absence of uncontrolled contaminants coming from solution. After this surface characterization, the influence of the different crystallographic orientations on the HER has been investigated in the same supporting electrolyte by opening the lower potential limit to slightly negative values. The current densities corresponding to the HER are presented as Tafel plots for each electrode in Figure 2. The solution in the working electrode compartment was quiescent, saturated with argon, and free of molecular hydrogen. Under these conditions, the cathodic Tafel plots are free from significant contributions from the anodic back reaction. It can be seen from Figure 2 that the Tafel plots are almost the same, irrespectively of the electrode orientation, under these experimental conditions, with a slope over 30 mV per decade as expected for platinum electrodes at low overpotentials. The rate of the HER was arbitrarily taken as the current density measured at E = 0 V and called io. The values of these rates and Tafel slopes for the three electrodes are summarized in Table I. The reaction rate and slope values are comparable to those presented in the literature for this reaction.17 These data show that the HER is insensitive to the crystallographic surface orientation in the potential range studied. Once the HER experiment was performed, the lower potential limit was changed again up to 0.06 V and voltammograms were recorded under the same conditions as in Figure 1 for each platinum electrode. Once the amount of hydrogen molecules remaining in solution as a consequence of the HER experiment was negligible, the voltammograms of Pt( 100) and Pt( 111) were coincident with those obtained immediately after the flame treatment. This proves the absence of significant contamination levels in the whole experiment. Moreover, the HER does not induce modifications on the electrode surface that could be detected electrochemically on the more packed platinum electrodes. As a difference with Pt( 100) and Pt( 111) electrodes, the HER induces small modifications on the characteristic voltammetric profile of the less closed packed electrode Pt(ll0) (Figure IC, dotted line). Although the overall electric charges do not change significantly, the peaks at 0.12 and 0.14 V become sharper, thus
Figure 1. Voltammograms of the three Pt basal orientations in 0.5 M HzS04: (a) Pt(ll1); (b) Pt(100); (c) Pt(ll0).
suggesting modifications in the population of the corresponding surface states.
Hydrogen Evolution on Pt
-1
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4771
t
-3
-
-4
-
-5
-
I
-0.04 -0.01
0
0.01
0.04
0.06
0.08
0.1
-2
-
-3
-
-4
-
-6
-
-Pt(Io0) S(11)
-7 +Pt(lll)
-liPt(ll0)
Figure 2. Log current density vs potential curves for the HER on the three Pt basal orientations.
\
-6 1
E(V/RHE)
1
-0.08 -0.06 -0.04-0.02 0
0.02 0.04 0.06 0.08
0.1
E(V/RHE)
Figure 4. Hydrogen evolution on a Bi-modified Pt( 100) electrode from a 0.5 M HzS04 hydrogen-freesolution. Tafel plots at different bismuth coverages: Os, = 0; 0.12; 0.29; 0.40; 0.48.
TABLE I: Kinetic Values of the HER for the Three Basal Platinum Orientations electrode Pt(ll1) Pt( 100) Pt(ll0)
lO3i0, A/cm*
0.84 0.84 0.97
b, mV/dec 30 31 30
simultaneously. So, care has to be taken in order to avoid the presence of such surface changes when an electrochemicalprocess is studied. These results can be used to check the electrode behavior in the whole experiment. The control of the electrode surface orientation as a result of the platinum sample preparation as well as the surface pretreatment ensures the character of structureinsensitive reaction of the HER in stable electrolyte solutions with a negligible low level of adsorbable impurities. Bi-Pt( 100). Figure 3 shows that the presence of irreversibly adsorbed bismuth adatoms on well-ordered Pt( 100) electrodes causes a diminutionof the hydrogen adsorption-desorptioncharge in the potential range below 0.45 V. The presence of the adatom on the surface can be also monitored by the redox process taking place at potentials higher than 0.8 V. The stability of the voltammetric profile upon cycling between carefully chosen potential limits13reflects the irreversibleadsorption of the adatom on the platinum substrate. It has been shown that the electric charge involved in the Hupd adsorption process decreases as the electric charge involved in the bismuth redox process increases.12 A linear plot is obtained with a -1 slope, fitting well with the relation
Figure 3. Stabilized voltammograms of a Pt( 100) surface with two different bismuth coverages. In (b), the arrow indicates the dissolution process.
It is noteworthy that modifications on the HU@process after the HER have been observed when working with initially welldefined surfaces that have been extensively cycled in the oxygen adsorption-desorption region. This pretreatment leads to the formation of randomly distributed surface defects and the HER can modify theinitial stateof theelectrode. At present, it remains unknown the effect - of such reorganization on the overall electrochemical response when a faradaic process takes place
It has been proposed that each bismuth adatom blocks two hydrogen adsorption sites and involves two electrons in its own redox process.I2 A monoelectronic process for bismuth would lead to a different relation as discussed below, and the hypothesis of the formation of Bi(II1) species has to be rejected because of the stability of the adatom layer. Relation 5 is very useful to control the presence of contamination. Contaminants would block the remaining hydrogen adsorption sites, thus diminishing QH, and the left-hand side of relation 5 would be lower than the righthand one. When this situation occasionally occurs the results are meaningless and have to be rejected. In the case when the irreversibly adsorbed species does not show a quantitatively welldefined redox process, this control cannot be monitored and the coverages have to be estimated by using the relation QH/QH'' only. In this cases the presence of undesired contaminants cannot be controlled and the adatom coverage can be overestimated.
4772 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993
Gdmez et al.
b (InV/d.C)
d
20
I
I
._
0.1
0
0.2
0.3
0.6
0.4
e,, Figure 5. Tafel slope of the HER vs bismuth coverage on Pt( IOO), in 0.5 M HzS04. IO' I ( E - o v ) ( A / ~ ~ ' )
Figure 7. Stabilized voltammograms of a Pt(100) surface with two different antimony coverages; (b) correspondsto &,b = 0.50 (all HU@sites blocked).
01 0
0.1
0.2
0.3
0.4
0.5
e,, Figure 6. Current density measured at 0 V for the hydrogen evolution as a function of the bismuth coverage on a Pt(100) surface.
Figure 4 shows the Tafel plots for the HER for various bismuth coverages ranging between 0 and 0.48. As discussed in the preceding paragraph, 6Bi = 0 represents the bare Pt( 100) surface, being the maximum bismuth coverage 0.5. It can be clearly seen that the Tafel plots are shifted toward more negative potentials by increasing 6Bi. This implies that bismuth acts like a poison, whose surface concentration can be controlled, lowering the rate of the HER. The linear part of the Tafel relation is followed up to current densities closed to 10-3 A/cm2. Figure 5 represents the values of the Tafel slope, b, vs 6Bi. It can be remarked that at low bismuth coverages a horizontal line, b = 3 1 mV/decade, is obtained. For bismuth coverages higher than 0.35440,a slight increase of the Tafel slopes is observed. In all cases the values of the slope are lower than 34 mV/decade. Figure 6 shows a plot of the current-density values measured at 0.0 V/RHE (io), as a measure of the HER rate, versus bismuth coverage. A similar plot is obtained for current density values measured at other constant potential values. The significant features of this plot are the following: (a) There is a first part in the curve, up to 6Bi = 0.35,where io diminishes linearly with 6Bi. The equation followed is
= jPt(O)(l - eBi) (6) where ipt(0) corresponds to the current density at 0.0 V/RHE for the bare Pt(100) electrode surface (OBi = 0).
(b) For 6gi > 0.35, the rate of the HER is strongly lowered. This coverage coincides with that corresponding to the increase of the Tafel slope as reported previously. Sb-Pt(100). Figure 7 shows the voltammetric behavior of two Pt( 100) electrodes with two different amounts of irreversibly adsorbed antimony. As in the preceding case, the adatom blocks the Hudsites and undergoes a redox process at potentials higher than 0.55 V. Although the redox process for this adatom takes place at lower potentials than for bismuth, there is still a good separation between the H,,d zone and this redox process. This means that the charge corresponding to both processes can be independently measured. The voltammetric profiles are stable and in this case the upper potential limit has not to be so precisely controlled as for bismuth adatoms. As in the case of bismuth, the inhibition of the H,,d adsorption process fits well with the relation12 QH
+ Qsb
QHR
(7)
The same conclusions can be reached with regard to the surface stoichiometry and contamination control. In Figure 8, the cathodic branches of the curves log(i) vs E for a series of coverages from the bare surface (6Sb = 0) up to 8Sb = 0.50 are shown. As it can be Observed, antimony also acts as a poison for the HER, increasing hydrogen overpotential. Figure 9 plots the Tafel slope vs &b. In a similar way to Bi poisoning, the slope is almost constant between eSb = 0 and 6Sb = 0.4and approximately equal to 31 mV/decade. For OSb > 0.4 the Tafel slope slightly increases. Figure 10shows the influence of an increaseof adatom coverage on the catalytic activity of the electrode. This behavior is similar to that of the Bi-Pt(l00) system. Up to eSb = 0.35 there is a
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4113
Hydrogen Evolution on Pt
-It
-0.08 -0.06 -0.04 -0.02
0
0.02 0.04 0.06 0.08
0
0.1
E(V/RHE) Figure 8. Hydrogen evolution on an Sb-modified Pt( 100)electrode from a 0.5 M H2S04 hydrogen-free solution. Tafel plots at different antimony coverages: &.b = 0; 0.10; 0.29; 0.40; 0.50. b(mVld.0)
20 .
1
10 1
0
0.1
0.2
0.3
0.4
0.6
cf, Figure 9. Tafel slope of the HER vs antimony coverage on Pt(100), in 0.5 M H2S04.
linear part that follows the equation For OSb > 0.4 the hydrogen evolution rate decreases in a strong way, being the inhibition effect greater than for Bi adatoms. It must be pointed out that, for a saturated surface (&, = OS), there still exists an important hydrogen evolution current:
(9)
Discussion Bare Surface. From the experimental results, it can be concluded that for the three basal platinum electrodes the Tafel slope is approximately equal to 30 mV/decade. So it can be thought that there is one common mechanism that is valid for all the bare surfaces at low overpotentials. This value is compatible with the generally accepted mechanism on platinum, which is considered to involve one initial discharge step in equilibrium followed by a recombination step that controls the rate of hydrogen evolution. On the other hand, the three basal platinum surfaces present the same current densityvalues for the HER, taking into account the experimental error range. The absence of foreign surface contaminants as well as random surface defects definitely proves
0.1
0.2
0.3
0.4
0.5
et. Figure 10. Current density measured at 0 V for the hydrogen evolution as a function of the antimony coverage on a Pt(100) surface.
that the HER is insensitive to the platinum surface orientation. Thus we can confirm that the hydrogen evolution reaction is not structure ~ensitive.~-~ In this respect possible implicationsof this fact could be considered. If the metallic surface is thought to be a packed array of rigid spheres, three types of surface sites can be distinguished. (a) Interstitial sites: adsorbed species bonding several surface metallicatoms. (b) Bridge sites: adsorbed species bonding two surface metallic atoms. (c) On-top sites: adsorbed species bonding one surface metallic atom. According to experimental results, to explain the non-structuresensitive character of the hydrogen evolution, an unspecific site of adsorption for the intermediate hydrogen atom involved in the evolution reaction (H*) should be postulated. It seems reasonable to establish that this siteshould have the same characteristics for the three basal orientations. Interstitial sites are clearly specific for each basal plane because they have different symmetry. This sites ought not to be considered good proposals for the adsorption of this species. However, interstitial sites are thought to be the H,,@ adsorption sites, because this adsorption process is strongly specific and dependent on the surface structure. Conversely, it can be suggested that the intermediate H* is adsorbed either on bridge sites or on top sites that are unspecific. This suggestion has been recently supported by other authors.6J8 Bi and Sb on F't(100). With regard to the poisoning effect of Bi and Sb on Pt(100) electrode surfaces, experimental results suggest that both adatoms behave in a similar way: (a) Tafel slopes remain approximatively constant in a wide range of adatom coverage (8Ad). Small deviations are observed only at surface concentrations near saturation (6Ad rr, 0.5). As a whole, the mechanism of the HER can be maintained in the case of these platinum-modified electrodes, which implies the recombination of the adsorbed hydrogen intermediate, H*, as a limiting step. (b) The electric charge involved in the adsorption-desorption process of the HU@decreaseslinearly with increasingthe amount of adatom. (c) The rate of the HER, arbitrarily measured as the current density at E = 0 V (io), decreaseslinearly with increasingadatom coverages following the equation that holds in a wide coverage range, 0 IeAd < 0.4. At higher coverage values, io decreasessteadily, but at saturation coverages, 8A.j = 0.5, the HER can still take place at a significant rate: 10% of that observed on clean Pt(100) for the case of Bi and 30% for Sb. In this study we deal with three different adsorbed species on the Pt(100) substrate: H u ~H*, , and the adatom (Bi or Sb). It
4114 The Journal of Physical Chemistry, Vol. 97,No. 18, 1993 seems reasonable to think that the experimental laws obtained should be a response of the surface arrangement of the different species in each case. The investigations of the adatom localization on the surface are difficult to get by other nonelectrochemical means, because there are few in situ data concerningthe geometricalarray formed on the surface by the adatom species. Although some data have been obtained by means of vacuum experiences,19,20the adlayer structure could be different in an electrochemical environment. The weight of the interactions in each medium could modify the characteristics of the adatom adsorption. We will first analyze the inhibition of H,,N adsorption on this orientation. If we divide expression 5 by QH" we will obtain
Gbmez et al.
W Figure 11. Proposed adsorption site for Bi adatoms on Pt(100). The interstitial Hu@sites are blocked.
If it is accepted that one surface Bi adatom exchanges two electrons in its surface redox process,13 we would write
QH=QH 'crit
and introducingthis expressionin (1 1) and reordering the resulting equation, we will obtain the inhibition law that expresses the exchangedcharge for the H uadsorption ~ as a function of adatom coverage: (13) = QHPt(l - 2eBi) and exactly the same expression is reached for the case of Sb adatoms. If the structure and orientation of the metallic electrodesurface are known and the adsorption site for H,,N is previously assumed (as we have mentioned the H ushould ~ be probably adsorbed on interstitial sites),we will beable to propose thenatureofadsorption sites for the adatom and the structure of the adlayer. If we want to contemplate all the possibilities to bond one adatom on a fcc(100) substrate, we will consider: (i) adatom coordinated to one substrate atom (on-top bond). (ii) adatom coordinated to two substrate atoms (bridge bond). (iii) adatom coordinated to four substrate atoms (interstitial bond). The final adlayer structure depends mainly on the lateral interactions between adatoms but also on the interaction adatomsubstrate:20J' (a) Iftherearenonet lateralinteractions,or thesearenegligible, adatoms will spread over the surface in an statistical and uniform way. The adlayer will be strongly disordered. This kind of distribution will be favored when the adatom species is very stable on the surface irrespectively of its adsorption site. (b) When net attractive interactions are present adatoms will tend to form compact bidimensional islands. In these islands there would be direct contact between adatoms (Ad-Ad bonds). These lateral attractive forces would control adlayer growth if interaction forces with the surface are not too important. (c) The system will present different ordered structures depending on the coverage if the net lateral interaction between adatoms is strongly repulsive. (d) If weak lateral interactions exist, the system will probably tend to form compact islands with an internal order whose structure would be similar to the saturation one. Each model will be characterized by an inhibition law.2' For the adelectrode under examination the possibilities (a) and (b) should be rejected because the former would be described by a potential inhibition law: QH
where it is implicitly accepted that the occupancy of vicinal sites has no influence on the entry of one new adspecies atom in an empty site. The latter will be characterized by an inhibition law:
where Ocrit corresponds to the coverage at which the adlayer has a hexagonal packed structure. It depends obviously on surface substrate atomic radius and on adatom radius. It also depends on thecrystal orientationof the substrate surface. The appropriate metal radii for predicting this closest-packedmonolayercoverages for such systems have been taken from ref 21. In the case of Bi = 0.66 and for the antimony adatom on a Pt( 100) surface = 0.85; we should obtain QH = eHPt(1- i s i e B i )
(16)
QH = QHPt(1 - 1.180,,)
(17)
This would imply that the surface could be divided into two fractions: a fractionofvalue 1-OAd/Ocit that is active for hydrogen evolution and a fraction eAd/&rit completely inactive. It is also clear that monoelectronic transfer in the redox surface reaction of the adatom cannot account for the experimental inhibition law as well as for the charges involved at saturation coverage. So, possibilities (a) and (b) must be excluded. We must study now if possibilities (c) and (d) agree with the experimental results. The corresponding inhibition law for both cases is = QHPt(l- neAJ
(18) which agrees with the experimental results. However, it is not possible to distinguish between both proposals. From the inhibition law, it is possible to establish the poisoning rangen = dOH/dOAd(number of hydrogen adsorption sites blocked by one adatom). OH is the coverage of Hu@and can be calculated from QH/QHP'. In the case under examination, this range, which corresponds to the coefficient of the coverage in experimental formula (18), is equal to two, as assumed previously. On the other hand, it can be seen that for @Ad = 0.5 the surface is completely blocked for H,,@ adsorption. With these data a model for the surface structure can be proposed with the following characteristics: H,pd would be adsorbed on interstitial sites, Bi and Sb adsorbed on bridge sites, and the saturation structure (OAd = 0.50) typec(2X2). Infact,ifbismuthorantimonyareadsorbed on bridge sites, each adatom affects two interstitial sites (Figure 11) and thus the poisoning range could be easily explained. It is interesting to point out that this conclusion can be reached by using only geometric arguments and this can be considered as a confirmation of the poor electronic effect that is expected for this adatom.20.23 The proposal of a saturation structure 4 2 x 2 ) can be held because the full coverage is 0.5 (which correspondsto the proposed structure) and for this coverage there are no interstitial sites that can be used for H,,@adsorption. If we assume that the HER takes place following the discharge QH
Hydrogen Evolution on Pt
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4775 c(2x2)
-
Figure 12. Lowering in the number of available pairs of Pt atoms for increasing adatom coverage. (2x2) 4 2 x 2 ) .
+ recombination mechanism we can write i = 2FkB&
(19) where kB is the kinetic rate constant assdated to the atomic recombination step and corresponds to the coverage of the H atom. This equation accepts the Langmuir isotherm for the intermediate H adsorption. When adatoms are introduced on the surface, the number of pairs of sites available for hydrogen adsorption lowers. This fact can be explicitly introduced if we consider the fraction of pairs of contiguous sites that are not blocked by adatoms,p. Then the expression for thecurrent density would become9J03
Obviously, p = 1 on the bare surface (8Ad = 0) and p = 1 - 8Ad for the linear part in Figures 6 and 10. To make an analysis of experimental results for the HER, it is necessary to previously fix the nature of the H* intermediate adsorption site. As we mention it will be assumed that hydrogen is on-top bonded. In the case of the system under scope, the inhibition law found implies thatp = 1 - 6Ad. This result excludes the possibility of a randomly distributed adlayer (possibility (a)). In the same manner the formation of compact bidimensional islands (possibility (b)) is also excluded because inhibition laws should be equal approximately to ipt(o)( 1 - 1 or ipt(o)( 1 1.289,). An ordered layer formation without distinguishing between possibilities (c) and (d) will be proposed. We can see at this point that the model accepted for the HU@ inhibition works well for explaining the HER poisoning. In this way, if weconsider the metallic adatom adsorption on bridge sites, it will be clear that one adatom will block one pair of Pt atoms involved in the intermediate hydrogen atom adsorption. This poisoning range is perfectly coherent with the variation of the fraction of pairs found in the experiences (p = 1 - 8Ad). It must be explained that for OAd > 0.35 there is an important decrease in the evolution current. This should be related to a strong lowering of the number of pairs. This can be seen in Figure 12, where two adlayer structures are presented. For example, when the system has initially a (2 X 2) structure and becomes a c(2 X 2) structure, an immediate effect on the number of available pairs can be observed. Surprisingly, for a saturationcoverage, the system still presents a no negligible evolution current while the Tafel slope, and therefore the basic mechanism for the HER, remains almost constant. This effect could be explained if we consider the possibility of evolution on sterically impeded pairs like those marked in Figure 13. A different possibility would consist of evolution on vicinal Bi or Sb adatoms but there are several reasons against this: (a) On metallic bismuth or antimony, hydrogen evolution reaction takes place at greater 0verpotentia1s.I~ (b) On metallic bismuth or antimony, the HER mechanism is not controlled by an atomic recombinationstep.17 (c) The minimum distance between Bi or Sb atoms in a 42x2) structure would be 2Ij2d(Pt-Pt). If we take a value d(Pt-Pt) = 2.7 A, the minimum
Bi-Pt(100)
Sb-Pt(100)
Figure 13. Sterically impeded pairs for Sb and Bi on Pt(100).
distance would be approximately equal to 3.8 A. With this distance the lateral interaction between hydrogen would become almost negligible and recombination would not be feasible. It is worthwhile to observe that for 8sb = 0.5 an important evolution current still remains that is three times the current observed for the system Bi-Pt( 100) at the same coverage. This could be also related to evolution on impeded pairs. Taking into account that the Sb atom is appreciably smaller than the Bi atom, it can be proposed that HER occurs more easily on these less impeded pairs (Figure 13). Adlayer stability has been checked after each experimental cycle by recording voltammetric curves. The charge involved in Hu@adsorption process does not vary after the HER and this fact confirms the idea that the adatoms are stable on the surface and they are not displaced from it. Only for 8Ad > 0.4 there is an unimportant loss of adatoms (1-2%) that could be related to evolution on impeded pairs. To summarize we will say that bismuth and antimony irreversibly adsorbson Pt( 100)orientation, originating an ordered adlayer. Each adatom would be coordinate to two surface platinum atoms (bridge adsorption mode). In this manner, two Hu@interstitial sites will be blocked per metallic adatom and, at the same time, one pair of available Pt atoms will be also blocked.
Conclusions (a) Experiments performed with well-ordered platinum electrodes with basal orientations in the absence of foreign surface contaminants prove that the HER is insensitive to the surface structure of the electrode. This strongly supports that the intermediate hydrogen atoms (H*) are adsorbed on unspecific sites. It has been confirmed that Hu@cannot be the intermediate involved in the HER. (b) Both chemisorbed bismuth and antimony strongly reduce the rate of the HER on a Pt(100) surface. This effect can be attributed to a blockage of intermediate hydrogen (H*) adsorption sites. There is a simultaneous effect on the Hu@sites. (c) The HER is not totally poisoned for coverage 8Ad = 0.5, at which there is no possibility of Hu@adsorption. This fact implies that Hu@cannot be the intermediate species involved in hydrogen evolution. On the other hand, it has been observed that the adatom inhibition range for both hydrogen atoms (Hu@and H*) is not the same. (d) For saturated surfaces (0 = OSO), HER is favored when the adatom species is Sb and this might be related to evolution on impeded pairs of Pt atoms. (e) Bismuth and antimony remain stable on the surface and there is no proof of their hypothetical mobility. ( f )Poisoning rangescould be explained by using only geometric reasons; it seems that there are no long-range electronic interactions. (8) In all cases the adlayer structureis ordered without random distributionsor bidimensional island formation. This fact should be a consequence of the existence of lateral repulsive forces between adatoms. These repulsive forces are favored because of
4176 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 the weakness of bonds Bi-Bi and Sb-Sb (these metals have a relatively small enthalpy of vaporization).
Acknowledgment. R.G. greatly acknowledges DGICYT (Ministerio de Educaci6n y Ciencia, Spain) for the award of a FPI grant. This work was financed by DGICYT (Projects CE910001 and PB90-0560). References and Notes ( I ) Bockris, J. O’M.; Reddy, A. K. N. Modern Electrochemistry; Plenum Press: New York, 1970; Vol. 2. (2) Vetter, K. J. Electrochemical Kinetics; Academic Press: London, 1967. (3) Conway, B. E.; Bai, L. J. Electroanal. Chem. 1986, 198, 149. (4) Yeager, E. J. Electrochem. SOC.1981, 128, 160C. (5) Seto, K.; Iannelli, A.; Love, B.; Lipkowski, J. J . Electroanal, Chem. 1987, 226, 351. (6) Kita, H.; Ye, S.;Gao, Y. J. Electroanal. Chem. 1992, 334, 351. (7) Schuldiner, S. J. Electrochem. SOC.1963,110,332; 1968,115,362. (8) Adzic, R. R. In Advances in Electrochemistry and Electrochemical Engineering, J. Wiley and Sons: New York, 1984.
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