- Email: [email protected]
Electrochemical behaviour of the Pt (111)-As system in acidic medium: adsorbed hydrogen and hydrogen reaction
193
J. Electroanal. Chem., 294 (1990) 193-208 Elsevier Sequoia S.A., Lausamte
Electrochemical behaviour of the Pt ( 111) -As system in acidic medium: adsorbed hydrogen and hydrogen reaction J. Clavilier Luboraroire d’Electrochimie Interfaciale du C.N.R.S.. 92195 Meudon PrincipaI Cedex (France)
J.M. Feliu, A. Femandez-Vega Departamento a’e Quimica-Frrica, 03080 Alicante (Spain)
I Place A. Brian4
and A. Ahlaz
Universidad de Alicante, Fact&ad de Ciencias, Apartado
99,
(Received 21 May 1990)
ABSTRACT
A detailed study of the deposition of arsenic on a platinum (111) oriented substrate was carried out by using either the technique of irreversible adsorption and transfer to 0.5 M H,SO, as the test electrolyte or classical UPD conditions. The relations between the two methods of layer formation were investigated. Adsorbed hydrogen was used to determine the stoichiometry of the arsenic surface compound formed and the hydrogen reaction for checking how compact the arsenic layer is. From this study it follows that classical adsorbed hydrogen cannot be considered as an intermediate in the hydrogen reaction. A discussion follows concernin g the position of the arsenic adatoms relative to the atoms of the substrate as a consequence of the absence of a correlation between the underpotentially adsorbed hydrogen and the hydrogen reaction.
(I) INTRODUCTION
It has been remarked that, when adatoms are irreversibly adsorbed on a platinum electrode, hydrogen adsorption on this surface can be completely suppressed, whereas hydrogen evolution is not so extensively affected by the presence of adatoms. This type of behaviour is at the origin of the idea that the hydrogen, which is underpotentiahy deposited, does not work as an intermediate in the hydrogen evolution reaction. Several papers have been published recently studying this aspect of the hydrogen reaction [l-3]. This paper deals with the study of arsenic adlayers on Pt (111) formed either in an irreversible adsorption process or in a classical UPD experiment. The stability, the compactness and the structure of the arsenic overlayers deposited by the two methods were checked by using both electrochemically 0022-0728/90/$03.50
6 1990 - Elsevier Sequoia S.A.
194
adsorbed hydrogen and the hydrogen reaction as surface probes. The use of well-defined platinum surfaces is expected to give a more precise physical meaning, particularly at the atomic level, to the electrochemical response of the various surface processes involved in the hydrogen reaction occurring on a platinum surface modified by adatoms. The result of the study is expected to give new insight into the classical hydrogen reaction as well as into the role of the surface structure and composition in electrocatalyst poisoning problems. (II) EXPERIMENTAL
The cell, electrodes, apparatus and techniques have been described elsewhere [4-71. Arsenic irreversible adsorption was carried out from 10e3 M As (III) in 0.5 M H,SO, as previously reported [4]. Arsenic UPD experiments were carried out in the usual way [8] with 10-5-10-3 M solutions of As (III) (from As,O,, Merck, p.a.) dissolved in the supporting electrolyte, 0.5 M H,SO,. Qualitative experiments to check hydrogen oxidation on arsenic-covered Pt (h, k,l) electrodes were carried out after bubbling N48 hydrogen through the solution for 2-3 min and recording the i/E curve in quiescent solutions. All experiments were done at room temperature and potentials were measured against a RHE. (III) RESULTS
(III. I) Irreversible
adsorption of arsenic on Pt (Ill)
The simple contact of Pt (111) with an acidic solution of As (III) leads to the formation of a stable monolayer or submonolayer or arsenic adspecies [6]. The presence and stability of these layers in a wide potential range can be characterized through the measurement of the free hydrogen adsorption sites, which can eventually reach a zero surface concentration when the amount of adatoms is sufficiently high. Simultaneously a corresponding stable voltammetric profile due to adsorbed arsenic may be obtained (Figs. la and lb), giving a well-defined reversible redox process in the range 0.4-0.7 V, which narrows when the arsenic surface amount is decreased simultaneously with the recovery of the hydrogen adsorption capability. It has been shown that this redox process involves the formation of an oxygenated arsenic compound on the Pt (111) surface [6]. From the two voltammetric features, the first one corresponding to the remaining hydrogen adsorption/desorption and the second one to the arsenic surface redox reaction, it is possible to determine independently the electric charge involved in each process. As a result, the hydrogen adsorption charge (Qn) versus the adatom oxidation charge (Q,) can be plotted. This plot is linear with a slope of - 1 (Fig. 2a). It is clear that the sum of the two charges amounts to a constant value which coincides with the hydrogen maximum adsorption capability (en)_ obtained with the pure Pt (111) surface in the test solution after the usual flame treatment. Also,
195
n
I/yA.cnr2
b
10
Fig. 1. Reversible voltammetric redox behaviour of irreversibly adsorbed arsenic on Pt (111) in 0.5 M HsSO,. (a) Adsorbed hydrogen electric charge Qu = 7 pC/cm*; adsorbed arsenic charge = 234 PC/cm2 measured at oxidation or reduction; 6” = 0.32. (b) Adsorbed hydrogen charge Qu =122 pC/cm2; adsorbed arsenic charge = 118 PC/cm*; 8,,, = 0.16. 50 mV/s in all figures.
0,
2sa
/UC
cm+
250
O,/pC
cni2
2oa
150
100
50
0
0
50
100
150 OAJ
200 C
cm+
J Io”a.
250
SO
100
1so
O,,/pC
cm?
Fig. 2. (a) Adsorbed hydrogen electric charge vs. the electric charge involved in the surface oxidation or reduction of arsenic irreversibly adsorbed on Pt (111). (b) Adsorbed hydrogen charge vs. the oxidation or reduction charge of bismuth irreversibly adsorbed on Pt (111).
196
the intersection of this straight line with the QAs axis leads to an amount of charge which can be easily assigned to the completion of a hydrogen blocking monolayer of arsenic on the Pt (111) surface. From these remarks the calculation of the arsenic adspecies coverage is obvious:
Q,.is+ QH = (Qdmax
(1)
Such a linear variation throughout the entire range of coverage is not an exception and may be observed with other irreversibly adsorbed species. For the sake of comparison, the results obtained with irreversibly adsorbed bismuth (Fig. 2b) are reported. Figures 2a and 2b show clearly a linear variation of the amount of adsorbed hydrogen against the amount of adatom for both bismuth and arsenic. The difference between the bismuth and arsenic adatoms is related to the slope values. QBi and Qn are related through the relation
KQBi + QH = (Q~>tmw.
(2)
with K being nearly 3/2. Discussion of the slope values in relations (1) and (2) may give a basis for a more precise picture of how the adatoms are bonded to the substrate. In this way, it has been proposed in ref. 7 that each bismuth adatom blocks three hydrogen sites and that two electrons are transferred in the redox surface reaction of bismuth. Figure 2b shows that this conclusion holds in the entire range of hydrogen coverage. Thus, it may be concluded that the bismuth adatoms on the surface keep the three-fold symmetry of the substrate surface in the same range. On the other hand, in the case of arsenic, it has been shown that after full desorption by electrochemical oxidation [6] the surface crystalline properties of the Pt (111) substrate were unchanged. Thus, arsenic adatoms do not induce substrate surface reconstruction or perturbation. In a way similar to bismuth, it may be assumed that the three-fold symmetry on the arsenic-modified surface is preserved. If this is the case, one arsenic atom could block three hydrogen sites on the Pt (111) electrode surface. Thus, the slope value of -1 in Fig. 2a means that the redox surface reaction of arsenic involves the exchange of three electrons per arsenic adatom [9]. This situation would agree with a surface arrangement for both arsenic and bismuth overlayers in an ordered (a x fi) R 30” surface lattice, as is often observed on Pt (111) surfaces [lO,ll] with many adatoms. This structure of the adatom overlayer agrees with that previously defined as a hydrogen blocking monolayer and corresponds to a coverage of l/3 relative to the platinum surface atoms of the Pt (111) 1 X 1 substrate. Larger amounts of arsenic than those required for complete blocking of the hydrogen adsorption sites may be adsorbed by applying the immersion technique. In this case, the surface redox process shows a multiplicity of peaks spreading in a broader region of potential (0.3-0.7 V) than in the case of a partial hydrogen coverage, as can be seen by comparing Figs. 1 and 3.
197
Hydrogen Fig. 3. (a) Voltammogram of irreversibly adsorbed arsenic on Pt (111); BAa= 0.41. ( -) adsorption charge recovery after one voltammetric sweep up to 1.2 V. (b) Voltammetric profile change of the arsenic redox reaction on Pt (111) with cycling from 0, = 0.38 to 0,, = 0.36.
The amount of charge involved in the adatom redox process may exceed that for which full hydrogen blocking is just obtained in Fig. 2a by up to 30%. This means that coverages higher than l/3 may be expected with Qn = 0. This is not surprising because when Qn reaches 0 the overlayer contains one arsenic atom per three atoms of the substrate. Because the radii of arsenic (0.148 MI) and platinum(0.l38 nm) are not very different [12], there is enough room between the arsenic adatoms for additional ones in the first layer. The question of the coverage at which the arsenic atoms could begin to adsorb in a second layer position will be discussed below. Despite the excess of arsenic, the layer can remain irreversibly adsorbed on the Pt (111) surface in a wide potential range over which at least two oxidation states of the adatom are detected: a zero valency state at a potential below the redox reaction and a three valency state above. However, when the higher potential limit is increased, another process can eventually be observed at 0.93 V (vs. RHE) (Fig. 3a). This process leads to adatom desorption and can be assigned to the formation of soluble arsenic(V) species [13].
198
This work does not deal with the As(III)-As(V) reaction, but this oxidation-dissolution process is used in the present work to control the arsenic coverage. So, it is possible to decrease the charge in the redox process of the adatom keeping the hydrogen charge almost zero on successive voltammetric cycles (Fig. 3b). This is manifested by the progressive disappearance of the oxidation step between 0.3 and 0.46 V. Only after this process has taken place, can increasing amounts of hydrogen adsorption be detected. When this situation is achieved, the electric charges involved in the surface processes for both species are distributed according to the linear plot in Fig. 2a.
Fig. 4. (a) Hydrogeu evolution reaction on Pt (111) with irreversibly adsorbed arsenic at high coverage in quiescent 0.5 M H,SO, after argon bubbling; 8,, = 0.37. Voltammetric cycle in a partial potential range of the same electrode showing the oxidation of mokcular hydrogen dissolved in the same quiescent solution (- - -). (b) Hydrogen evolution reaction on a pure Pt (111) surface as in (a).
199
The striking feature with this kind of surface modified by arsenic adspecies is its behaviour relative to the hydrogen evolution reaction. Even with surfaces (Fig. 4a) where no hydrogen adsorption is observed, the evolution of hydrogen occurs in the same potential range as on pure Pt (ill), with all its adsorption sites occupied by hydrogen adatoms (Fig. 4b). On the other hand, molecular hydrogen present in the solution can be oxidized on the Pt (Ill)-As surface fully blocked for hydrogen adsorption as well as on pure Pt (111) (Fig. 4b). These results for the hydrogen reactions confirm that underpotentially adsorbed hydrogen does not seem to be directly involved either in the hydrogen evolution or in the hydrogen oxidation reaction. In order to check the range of the arsenic surface amount which is able to block the reaction of hydrogen, it may be adsorbed under potential control in the usual way of a UPD experiment. (III.2)
How can UPD of arsenic on Pt (111) be studied?
(III.2.1) Classical UPD conditions When arsenic is electrodeposited on Pt (111) from a 10e3 M As(II1) acid solution, a. typical concentration for correct observation of UPD of metals on metals with a sweep rate of 50 mV s-l, the voltammogram does not show the features observed with irreversibly adsorbed arsenic. The results obtained with various potential limits and successive cycles are reported in Fig. 5. In Fig. 5a, the curve reported for a flame-cleaned electrode immediately after contact with the solution at 0.6 V does not show the reversible rcdox surface reaction. Only the first reduction sweep shows a peak at 0.38 V, which cannot be identified with the reduction peak of the irreversibly adsorbed arsenic previously recorded at 0.55 V. The featureless second sweep is characteristic of an ill-defined and poorly reactive surface compound. In Fig. 5b, the voltammogram shows the effect of the increase of the upper potential limit. The oxidation peak above 0.9 V corresponds to the oxidation of As(II1) to As(V). This current is due mainly to oxidation of the arsenic in solution. Nevertheless, it has been remarked in Fig. 3a that at this potential, irreversibly adsorbed As(II1) was also oxidized and desorbed. An important point in the case of the polarization limits chosen in Fig. 5b is that, despite the oxidation of As(II1) to As(V), arsenic is always present on the platinum surface. From the window-opening effect at the lower potential limit (Fig. 5c) on the height of the reduction peak at 0.46 V it may be stated that the reduction current below 0.3 V corresponds to the deposition of arsenic. Thus, the peak at 0.46 V corresponds to the oxidation of As(O) to As(II1) as a soluble compound, while the As(II1) to As(V) reaction takes place above 0.9 V. From the measurement of the electric charge, it is clear that a multilayer of arsenic may be deposited in this way. Thus, for the system under consideration, the conditions for normal UPD do not allow the observation of a submonolayer or
200
a
VpA.cm-’ 460.
b
240.
0
0.1
Fig. 5. (a) The first three vokammetric sweeps between 0.3 and 0.8 V of flame-cleaned Pt (111) in 0.5 M H,SO, + 1O-3 M As(II1). (b) () Stationary voltammogram between 0.3 and 0.86 V after (a). (__) Oxidation of As(U) to As(V) above 0.9 V. (c) Window-opening experiment showing bulk arsenic deposition and inhibition of the hydrogen evolution reaction.
monolayer, but only thick film deposition. None of the voltammograms reported in Fig. 5 presents the characteristic reversible surface redox reaction observed with irreversibly adsorbed arsenic.
201
(111.2.2) Arsenic deposition from 10 - 5 M As(III) solution It is clear that the irreversible adsorption of arsenic is an important complication in the classical UPD concept when the adsorbable species is electrodeposited in equilibrium from a solution of its ions at a given concentration.
Fig. 6. (a) Evolution of the voltammogram profile of a flame-cleaned Pt (111) surface in contact with 0.5 M H,SO, + 10s5 M As(II1). Initial potential: 0.75 V. First (- . -), fifth ( -), tenth (- - -) and 20th (- - -) cycles showing the progressive disappearance of the surface redox reaction. (b) Effect of the ) Stationary profile after (a) with E, = 0.7 or 0.8 V; increase of the upper potential Iimit E,. ((- . -) Eu = 1.0 V, (- - -) EU = 1.05 V showing the progressive appearance of the surface redox reaction.
202
In order to understand better how the irreversible adsorption process is masked and mixed with the deposition experiment, it is necessary to form in a more progressive manner the multilayered surface compound. This may be achieved by working with a more dilute As(II1) solution and by recording the effect of the accumulation of arsenic at the surface on the redox surface reaction of irreversibly adsorbed arsenic. It is also interesting to consider the reverse effect of dissolution of an excess of accumulated arsenic on this redox reaction using various procedures. Therefore, successive voltammetric cycles were recorded in a lop5 M As(II1) after flame treatment of the electrode by choosing the upper potential limit to avoid oxidation of As(II1) to As(V). The voltammetric profile changes continuously, including the number of peaks (Fig. 6a). Noteworthy is the appearance of a reduction peak at 0.18 V which becomes more and more marked upon cycling. This peak corresponds to the bulk deposition of arsenic, as has been shown from deposition with higher arsenic concentrations. The electric charge for the different voltammograms increases with the number of cycles until a constant value of nearly 270 &/cm’ is obtained irrespective of the voltammogram profile. However, the charge transferred in the potential range corresponding to the redox process of the irreversibly adsorbed arsenic first increases with the cycling to a maximum value and then vanishes. It can be shown that arsenic dissolution occurs only when the As(III)-As(V) reaction takes place. This is achieved by putting the upper potential limit above the potential of the As(III)-As(V) reaction. Then, after some cycles up to 1 V, the arsenic accumulated in the previous experiment is partially dissolved and the inverse transformation to that reported in Fig. 6a may be observed. The pair of peaks in the potential region around 0.55 V appears progressively when arsenic is dissolved (Fig. 6b). These experiments show that (i) the excess of arsenic masks the redox process of the irreversibly adsorbed adatom; (ii) the amount of arsenic on the surface also depends on the upper potential limit; and (iii) As(V) is formed from the deposited arsenic which is then dissolved as in the irreversible adsorption experiments. These facts imply that the arsenic coverage cannot be determined in the classical UPD concept. However, the evolution of the voltammetric profile suggests a criterion for the detection of the building of the second layer, i.e. when the redox surface reaction of irreversibly adsorbed arsenic loses its sharp voltammetric profile. (III.2.3) Hydrogen reaction and arsenic deposition In arsenic deposition experiments from a 1O-3 M solution, in contrast to experiments with irreversibly adsorbed arsenic, the hydrogen evolution reaction is considerably inhibited: the reaction takes place at negative potential values versus a RHE on the surface covered by thick arsenic films (Fig. 5~). So, this experiment shows that hydrogen evolution on multi-layered (bulk) arsenic is highly inhibited relative to pure Pt (111). An attempt at the oxidation of dissolved molecular hydrogen was unsuccessful in the 10e3 M arsenic solution (Fig. 5b) at a potential of 0.3 V on a Pt (111) surface cycled several times from 0.3 to 1.1 V avoiding bulk arsenic deposition. The absence
203
of an oxidation current implies complete modification of the hydrogen dissociation properties of the surface. This fact also agrees with the previous assumption that under classical UPD conditions the amount of adsorbed arsenic reaches values higher than that required for complete blocking of the hydrogen adsorption sites. This result provides evidence for the origin of the impossibility of control of the surface layer composition in the submonolayer range with this type of deposition experiment in contrast to irreversible adsorption, and shows clearly the formation of at least a compact arsenic layer before the deposition at 0.3 V takes place forming a multilayered arsenic film. (111.2.4) Stability of arsenic after classical UPD conditions The stability of the arsenic multilayer can be tested by the following experiment. An arsenic multilayered Pt (111) electrode is taken out of the arsenic solution at 0.4 V, rinsed and introduced into the test solution containing hydrogen. The appearance of a hydrogen oxidation current (Fig. 7) and the observation of a redox process as in the case of the highest coverages in the irreversible adsorption experiments mean that, with respect to the arsenic amount reported in Fig. 5, some arsenic has been dissolved from the surface. However, the surface remains completely blocked for hydrogen adsorption. This experiment contrasts with the irreversible adsorption results where the arsenic adlayer was found to be stable under the same experimental conditions. It may be concluded that it is this part of the irreversibly adsorbed arsenic which stays on the surface after the transfer of the arsenic multilayered electrode. These two different behaviours of the arsenic adlayer indicate that two kinds of arsenic adatoms exist: a stable adlayer of irreversibly adsorbed arsenic on Pt (111) sites, and arsenic atoms adsorbed in excess forming agregates or multilayers. The
Fig. 7. Voltammogram of a Pt (lll>modified electrode obtained after arsenic deposition from 10W3 M As(III), rinsed and transferred into 0.5 M H,SO, after deaeration by argon bubbling. (1) shows the blocking of the hydrogen adsorption; (2) shows the arsenic surface redox reaction; (3) shows molecular hydrogen oxidation in the same solution after hydrogen bubbling.
204
dissolution of the latter could be due to oxidation by oxygen dissolved in the water used for rinsing. (111.3) Hydrogen evolution on Pt (I II) electrodes precovered arsenic
with irreversibly adsorbed
Arsenic-precovered Pt (111) electrodes were prepared at different coverages ranging from 0 to 0.45 as described in Section (111.1). As has been pointed out, hydrogen evolution in the absence of dissolved hydrogen takes place in the same potential range at a surface blocked or not for hydrogen adsorption by irreversibly adsorbed arsenic. So, the Tafel slopes under such conditions for an arsenic-precovered Pt (111) electrode as well as for a pure Pt (111) surface were determined. This contrasts with the case of UPD experiments where the hydrogen evolution reaction was clearly inhibited as a consequence of the forced adsorption of arsenic. The Tafel plots give a constant slope value of 30 mV (Fig. 8) for all the arsenic coverages, ranging from 0 up to the completion of the stable adlayer which completely blocks hydrogen adsorption (6”, = l/3). With higher surface amounts of irreversibly adsorbed arsenic, 6,, > l/3, the reaction slows down at high current densities and the Tafel slope is 30 mV at low current densities only. It has been verified that the slope was independent of the sweep rate in a range including that used in Fig. 8, i.e. from 5 to 50 mV/s.
Fig. 8. Hydrogen evolution on an As-modified Pt (111) electrode from 0.5 M H,SO, hydrogen-free solution. Tafel plots at different arsenic coverages: 8,, = 0 (0); 0.15 (X); 0.22 (A); 0.4 (0); 0.43 (V). 10 mV/s.
205
i,A lo+
cm“
y
Fig. 9. Current density measured at 0 V for the hydrogen evolution reaction vs. wsenic coverage on the Pt (111) surface.
Figure 9 shows the decrease of the current density of the hydrogen evolution reaction at 0 V as a function of the arsenic coverage. This graph has two sections. There is a clear change in the rate of the current density decrease with arsenic coverage when complete inhibition of the hydrogen adsorption monolayer is reached (e,, = l/3). At lower coverages, the current densities decrease slowly, but at higher coverages there is a sudden fall in the current together with an increase in the Tafel slope, as deduced from Fig. 8. It is important to remark that some experiments carried out with the same technique lead to current density values lower than those reported in Fig. 9. Two such points have been plotted in Fig. 9 as open circles. In the corresponding cases, the sum QAs + Qu was always found to be lower than 240 @/cm* (around 220 @Z/cm*). The deviation in the electrical charge indicates that an occasional surface contamination occurred in the course of the experiment. This conclusion agrees with the low current density found. These points can be considered as irrelevant but the following conclusions may be emphasized: (a) in order to check the surface cleanliness, both the hydrogen and the adatom electric charges must be measured with their sum equal to 240 PC/cm*; (b) the contamination affects mainly the determination of the hydrogen coverage with two consequences: an increase in the number of electrons per site for the surface redox reaction, yielding an apparent value, and a decrease in the effective hydrogen evolution current. So, the presence of arsenic on the Pt (111) surface does not induce any dramatic change in the kinetics of the hydrogen evolution reaction until 0,, = l/3. It is interesting to note that the current density at 6,, = l/3 is two-thirds that measured at 0,, = 0. This may be correlated with the stoichiometry of one arsenic atom per three surface platinum atoms found at this particular coverage. This result is
206
unexpected because the current does not fall with a quadratic (1 - 8,) as in other cases [2,8].
dependence
on
(IV) DISCUSSION AND CONCLUSION
The main results of this work may be summarized as follows. The decrease in the adsorption capability of platinum down to zero with increasing amounts of adsorbed arsenic up to 0,, = l/3 is accompanied by a decrease in the current density of hydrogen evolution which is nearly proportional to the arsenic coverage with a constant Tafel slope of 30 mV. In the same range of arsenic coverage, molecular hydrogen may be oxidized too. Higher arsenic coverages may be reached; then the hydrogen evolution reaction rate decreases more rapidly and the hydrogen reaction is blocked when the arsenic multilayer builds up. The effect of arsenic adsorption at a,, = l/3 on the hydrogen evolution rate when hydrogen adsorption is zero indicates that the underpotentially (UPD) adsorbed hydrogen can be considered as a species independent of the mechanism of hydrogen evolution, This conclusion was also reached by other authors using other experimental approaches [l-3]. This characteristic coverage implies that platinum atoms are not available for UPD adsorbed hydrogen, but are available for hydrogen recombination or hydrogen dissociation, the latter implying a hydrogen adsorption step with an adsorbed hydrogen intermediate which cannot be of the UPD type. These apparently conflicting results may be reconciled by considering the position of the arsenic adatoms with respect to that of the substrate atoms. Let us consider the two possible positions an arsenic adatom may occupy on a Pt (111) surface which may be hardly distinguished by the LEED technique. The first one is the hollow position (octahedral site). In this case, an arsenic atom has three platinum atoms in the position of first nearest neighbours as shown in Fig. 10a. When the (fi X &)R 30” adlayer is completed, all the platinum atoms are bonded to an arsenic atom and are homogeneously influenced by the presence of arsenic adatoms. The second position to be considered is the on-top position. In this case, presented in Fig. lob, every arsenic adatom has only one platinum atom in the position of first nearest neighbour. When the structure (6 x 6) R 30” is completed, only one-third of the surface platinum atoms are bonded to an arsenic atom; the other two-thirds correspond to platinum atoms with orbitals free for adsorption bonding. With the on-top model, it can be seen that the octahedral sites have one of the three platinum surface atoms bonded to an arsenic atom. This breakdown of the site symmetry may explain the blocking of hydrogen adsorption on the site. At 0,, = l/3, all the octahedral sites are thus affected by the presence of arsenic adatoms and hydrogen adsorption could be completely inhibited. In this surface structure, the platinum atoms which are not in the position of first nearest neighbour are less affected by the presence of adsorbed arsenic and their three-fold symmetry is preserved. They could be the platinum atoms which participate in the hydrogen recombination or molecular hydrogen dissociation in the hydrogen reac-
a
b
Fig. 10. Hard sphere model for the (& X 6) R 30° structure of an overlayer of heteroatoms (A) on a (111) foe substrate. (a) A in hollow position. (b) A in on-top position. (1) refers to the substrate atoms in the position of fist nearest neighbours relative to A. (2) refers to the second nearest neighbour. (0) position of the octahedral sites around A.
tion. This model could explain the decrease in the hydrogen evolution current (Fig. 9), when the (6 x 6) R 30” overlayer is forming, down to a value which is two-thirds of its initial value at completion of the overlayer. The change of slope above 6,, = l/3 shows that the adatoms adsorbed at a higher coverage are more efficient for the inhibition of the hydrogen evolution reaction while they cannot have any additional effect on hydrogen adsorption which is fully blocked. The effect on the hydrogen evolution reaction may be tentatively explained by assuming that an arsenic atom adsorbing at such a coverage, in addition to its own inhibiting effect, increases the inhibiting effect of the arsenic atoms already adsorbed. A possibility would be that the latter increase their number of first nearest neighbours. In contrast to the previous conclusions drawn for the on-top model, the hollow model suggests that both the UPD hydrogen blocking and the inhibition of the hydrogen evolution reaction could change to the same extent with arsenic coverage, If it can be definitely proved that a surface platinum atom in the position of first nearest neighbour with an arsenic adatom is inactive for hydrogen recombination or dissociation, then the hollow model can be excluded in accounting for the present results and the on-top model works. in the latter, evidently the hydrogen intermediate, which is an adsorbed species, implies that platinum atoms give rise to a type of adsorption site different from that used for UPD adsorbed hydrogen. This conclusion agrees with that in ref. 3. This remains an open question for which further work is required. This may be considered as a qualitative approach which tries to reconcile the hydrogen adsorption properties of platinum and the well-known hydrogen reaction in which a type of adsorbed hydrogen different from the UPD one is involved as an intermediate. In the case of well-defined platinum electrodes
208
such as Pt (ill), the microscopic aspect of the problem can be better defined and there is now a sufficient amount of data with Pt (ill)-adatom systems to think that the observations reported in this paper and the attempt at explaining the behaviour of the system may be generalized to other irreversible adsorbed adatoms using both UPD adsorbed hydrogen and the hydrogen reaction as surface probes. ACKNOWLEDGEMENTS
We are grateful to Dr. D. Aberdam for helpful ~~ussions. One of us (J.C.) ackuowledges DGYCIT de1 Ministerio de Education y Ciencia (Spain) and Direction de la Cooperation Scientifique et Technique du Ministere de Affaires Etrangeres (France) for a 3-month stay at the Department of Physical Chemistry of the University of Alicante as part of the Mercure programme. The financial support of the DGICYT through project No. PBS7-0795 is also gratefully acknowledged. REFE~NCXS 1 2 3 4 5 6 7 8 9 10 11 12 13
B.E. Conway, Sci. Frog. Oxf., 71 (1987) 479. E. pllotopopoff and P. Marcus, J. Vat. Sci. Technol., 5 (1987) 944. R.J. Nichols and A. Bewick, J. Electroam& Chem., 243 (1988) 445. J. Clavilier, R. Faure, G. Guinet and R. Durand, J. EIectroanai. Cbem., 107 (1980) 205. J. Clavilier, J. Etectroanal. Chem., 107 (1980) 211. J.M. Feliu, A. Fernandez-Vega, A. Aldaz and J. Clavilier, J. Electroanal. Chem., 256 (1988) 149. J. Clavilier, J.M. Feliu and A. Aldaz, J. Electroanai. Cbem., 243 (1988) 419. RR. Adz& in H. Gerischer (Ed.), Advances in Ekxtrocbemistry and EiectrochemicaI Engineezing, Vol. 13, Wiley, New York, 1984, p. 159. N. Furuya and S. Motoo, J. Electroanal. Chem., 78 (1977) 243. G.A. Somotjai, J. Chem. Phys., 54 (1971) 389. M.T. Paffett, CT. Campbell and T.N. Taylor, J. Chem. phys., 85 (1986) 6176. N. Furuya and S. Motoo, J. E%x~roanai.Chem., 98 (1979) 189. T. Loucka, J. EIectroanaI. Chem., 47 (1973) 103.
J. Electroanal. Chem., 294 (1990) 193-208 Elsevier Sequoia S.A., Lausamte
Electrochemical behaviour of the Pt ( 111) -As system in acidic medium: adsorbed hydrogen and hydrogen reaction J. Clavilier Luboraroire d’Electrochimie Interfaciale du C.N.R.S.. 92195 Meudon PrincipaI Cedex (France)
J.M. Feliu, A. Femandez-Vega Departamento a’e Quimica-Frrica, 03080 Alicante (Spain)
I Place A. Brian4
and A. Ahlaz
Universidad de Alicante, Fact&ad de Ciencias, Apartado
99,
(Received 21 May 1990)
ABSTRACT
A detailed study of the deposition of arsenic on a platinum (111) oriented substrate was carried out by using either the technique of irreversible adsorption and transfer to 0.5 M H,SO, as the test electrolyte or classical UPD conditions. The relations between the two methods of layer formation were investigated. Adsorbed hydrogen was used to determine the stoichiometry of the arsenic surface compound formed and the hydrogen reaction for checking how compact the arsenic layer is. From this study it follows that classical adsorbed hydrogen cannot be considered as an intermediate in the hydrogen reaction. A discussion follows concernin g the position of the arsenic adatoms relative to the atoms of the substrate as a consequence of the absence of a correlation between the underpotentially adsorbed hydrogen and the hydrogen reaction.
(I) INTRODUCTION
It has been remarked that, when adatoms are irreversibly adsorbed on a platinum electrode, hydrogen adsorption on this surface can be completely suppressed, whereas hydrogen evolution is not so extensively affected by the presence of adatoms. This type of behaviour is at the origin of the idea that the hydrogen, which is underpotentiahy deposited, does not work as an intermediate in the hydrogen evolution reaction. Several papers have been published recently studying this aspect of the hydrogen reaction [l-3]. This paper deals with the study of arsenic adlayers on Pt (111) formed either in an irreversible adsorption process or in a classical UPD experiment. The stability, the compactness and the structure of the arsenic overlayers deposited by the two methods were checked by using both electrochemically 0022-0728/90/$03.50
6 1990 - Elsevier Sequoia S.A.
194
adsorbed hydrogen and the hydrogen reaction as surface probes. The use of well-defined platinum surfaces is expected to give a more precise physical meaning, particularly at the atomic level, to the electrochemical response of the various surface processes involved in the hydrogen reaction occurring on a platinum surface modified by adatoms. The result of the study is expected to give new insight into the classical hydrogen reaction as well as into the role of the surface structure and composition in electrocatalyst poisoning problems. (II) EXPERIMENTAL
The cell, electrodes, apparatus and techniques have been described elsewhere [4-71. Arsenic irreversible adsorption was carried out from 10e3 M As (III) in 0.5 M H,SO, as previously reported [4]. Arsenic UPD experiments were carried out in the usual way [8] with 10-5-10-3 M solutions of As (III) (from As,O,, Merck, p.a.) dissolved in the supporting electrolyte, 0.5 M H,SO,. Qualitative experiments to check hydrogen oxidation on arsenic-covered Pt (h, k,l) electrodes were carried out after bubbling N48 hydrogen through the solution for 2-3 min and recording the i/E curve in quiescent solutions. All experiments were done at room temperature and potentials were measured against a RHE. (III) RESULTS
(III. I) Irreversible
adsorption of arsenic on Pt (Ill)
The simple contact of Pt (111) with an acidic solution of As (III) leads to the formation of a stable monolayer or submonolayer or arsenic adspecies [6]. The presence and stability of these layers in a wide potential range can be characterized through the measurement of the free hydrogen adsorption sites, which can eventually reach a zero surface concentration when the amount of adatoms is sufficiently high. Simultaneously a corresponding stable voltammetric profile due to adsorbed arsenic may be obtained (Figs. la and lb), giving a well-defined reversible redox process in the range 0.4-0.7 V, which narrows when the arsenic surface amount is decreased simultaneously with the recovery of the hydrogen adsorption capability. It has been shown that this redox process involves the formation of an oxygenated arsenic compound on the Pt (111) surface [6]. From the two voltammetric features, the first one corresponding to the remaining hydrogen adsorption/desorption and the second one to the arsenic surface redox reaction, it is possible to determine independently the electric charge involved in each process. As a result, the hydrogen adsorption charge (Qn) versus the adatom oxidation charge (Q,) can be plotted. This plot is linear with a slope of - 1 (Fig. 2a). It is clear that the sum of the two charges amounts to a constant value which coincides with the hydrogen maximum adsorption capability (en)_ obtained with the pure Pt (111) surface in the test solution after the usual flame treatment. Also,
195
n
I/yA.cnr2
b
10
Fig. 1. Reversible voltammetric redox behaviour of irreversibly adsorbed arsenic on Pt (111) in 0.5 M HsSO,. (a) Adsorbed hydrogen electric charge Qu = 7 pC/cm*; adsorbed arsenic charge = 234 PC/cm2 measured at oxidation or reduction; 6” = 0.32. (b) Adsorbed hydrogen charge Qu =122 pC/cm2; adsorbed arsenic charge = 118 PC/cm*; 8,,, = 0.16. 50 mV/s in all figures.
0,
2sa
/UC
cm+
250
O,/pC
cni2
2oa
150
100
50
0
0
50
100
150 OAJ
200 C
cm+
J Io”a.
250
SO
100
1so
O,,/pC
cm?
Fig. 2. (a) Adsorbed hydrogen electric charge vs. the electric charge involved in the surface oxidation or reduction of arsenic irreversibly adsorbed on Pt (111). (b) Adsorbed hydrogen charge vs. the oxidation or reduction charge of bismuth irreversibly adsorbed on Pt (111).
196
the intersection of this straight line with the QAs axis leads to an amount of charge which can be easily assigned to the completion of a hydrogen blocking monolayer of arsenic on the Pt (111) surface. From these remarks the calculation of the arsenic adspecies coverage is obvious:
Q,.is+ QH = (Qdmax
(1)
Such a linear variation throughout the entire range of coverage is not an exception and may be observed with other irreversibly adsorbed species. For the sake of comparison, the results obtained with irreversibly adsorbed bismuth (Fig. 2b) are reported. Figures 2a and 2b show clearly a linear variation of the amount of adsorbed hydrogen against the amount of adatom for both bismuth and arsenic. The difference between the bismuth and arsenic adatoms is related to the slope values. QBi and Qn are related through the relation
KQBi + QH = (Q~>tmw.
(2)
with K being nearly 3/2. Discussion of the slope values in relations (1) and (2) may give a basis for a more precise picture of how the adatoms are bonded to the substrate. In this way, it has been proposed in ref. 7 that each bismuth adatom blocks three hydrogen sites and that two electrons are transferred in the redox surface reaction of bismuth. Figure 2b shows that this conclusion holds in the entire range of hydrogen coverage. Thus, it may be concluded that the bismuth adatoms on the surface keep the three-fold symmetry of the substrate surface in the same range. On the other hand, in the case of arsenic, it has been shown that after full desorption by electrochemical oxidation [6] the surface crystalline properties of the Pt (111) substrate were unchanged. Thus, arsenic adatoms do not induce substrate surface reconstruction or perturbation. In a way similar to bismuth, it may be assumed that the three-fold symmetry on the arsenic-modified surface is preserved. If this is the case, one arsenic atom could block three hydrogen sites on the Pt (111) electrode surface. Thus, the slope value of -1 in Fig. 2a means that the redox surface reaction of arsenic involves the exchange of three electrons per arsenic adatom [9]. This situation would agree with a surface arrangement for both arsenic and bismuth overlayers in an ordered (a x fi) R 30” surface lattice, as is often observed on Pt (111) surfaces [lO,ll] with many adatoms. This structure of the adatom overlayer agrees with that previously defined as a hydrogen blocking monolayer and corresponds to a coverage of l/3 relative to the platinum surface atoms of the Pt (111) 1 X 1 substrate. Larger amounts of arsenic than those required for complete blocking of the hydrogen adsorption sites may be adsorbed by applying the immersion technique. In this case, the surface redox process shows a multiplicity of peaks spreading in a broader region of potential (0.3-0.7 V) than in the case of a partial hydrogen coverage, as can be seen by comparing Figs. 1 and 3.
197
Hydrogen Fig. 3. (a) Voltammogram of irreversibly adsorbed arsenic on Pt (111); BAa= 0.41. ( -) adsorption charge recovery after one voltammetric sweep up to 1.2 V. (b) Voltammetric profile change of the arsenic redox reaction on Pt (111) with cycling from 0, = 0.38 to 0,, = 0.36.
The amount of charge involved in the adatom redox process may exceed that for which full hydrogen blocking is just obtained in Fig. 2a by up to 30%. This means that coverages higher than l/3 may be expected with Qn = 0. This is not surprising because when Qn reaches 0 the overlayer contains one arsenic atom per three atoms of the substrate. Because the radii of arsenic (0.148 MI) and platinum(0.l38 nm) are not very different [12], there is enough room between the arsenic adatoms for additional ones in the first layer. The question of the coverage at which the arsenic atoms could begin to adsorb in a second layer position will be discussed below. Despite the excess of arsenic, the layer can remain irreversibly adsorbed on the Pt (111) surface in a wide potential range over which at least two oxidation states of the adatom are detected: a zero valency state at a potential below the redox reaction and a three valency state above. However, when the higher potential limit is increased, another process can eventually be observed at 0.93 V (vs. RHE) (Fig. 3a). This process leads to adatom desorption and can be assigned to the formation of soluble arsenic(V) species [13].
198
This work does not deal with the As(III)-As(V) reaction, but this oxidation-dissolution process is used in the present work to control the arsenic coverage. So, it is possible to decrease the charge in the redox process of the adatom keeping the hydrogen charge almost zero on successive voltammetric cycles (Fig. 3b). This is manifested by the progressive disappearance of the oxidation step between 0.3 and 0.46 V. Only after this process has taken place, can increasing amounts of hydrogen adsorption be detected. When this situation is achieved, the electric charges involved in the surface processes for both species are distributed according to the linear plot in Fig. 2a.
Fig. 4. (a) Hydrogeu evolution reaction on Pt (111) with irreversibly adsorbed arsenic at high coverage in quiescent 0.5 M H,SO, after argon bubbling; 8,, = 0.37. Voltammetric cycle in a partial potential range of the same electrode showing the oxidation of mokcular hydrogen dissolved in the same quiescent solution (- - -). (b) Hydrogen evolution reaction on a pure Pt (111) surface as in (a).
199
The striking feature with this kind of surface modified by arsenic adspecies is its behaviour relative to the hydrogen evolution reaction. Even with surfaces (Fig. 4a) where no hydrogen adsorption is observed, the evolution of hydrogen occurs in the same potential range as on pure Pt (ill), with all its adsorption sites occupied by hydrogen adatoms (Fig. 4b). On the other hand, molecular hydrogen present in the solution can be oxidized on the Pt (Ill)-As surface fully blocked for hydrogen adsorption as well as on pure Pt (111) (Fig. 4b). These results for the hydrogen reactions confirm that underpotentially adsorbed hydrogen does not seem to be directly involved either in the hydrogen evolution or in the hydrogen oxidation reaction. In order to check the range of the arsenic surface amount which is able to block the reaction of hydrogen, it may be adsorbed under potential control in the usual way of a UPD experiment. (III.2)
How can UPD of arsenic on Pt (111) be studied?
(III.2.1) Classical UPD conditions When arsenic is electrodeposited on Pt (111) from a 10e3 M As(II1) acid solution, a. typical concentration for correct observation of UPD of metals on metals with a sweep rate of 50 mV s-l, the voltammogram does not show the features observed with irreversibly adsorbed arsenic. The results obtained with various potential limits and successive cycles are reported in Fig. 5. In Fig. 5a, the curve reported for a flame-cleaned electrode immediately after contact with the solution at 0.6 V does not show the reversible rcdox surface reaction. Only the first reduction sweep shows a peak at 0.38 V, which cannot be identified with the reduction peak of the irreversibly adsorbed arsenic previously recorded at 0.55 V. The featureless second sweep is characteristic of an ill-defined and poorly reactive surface compound. In Fig. 5b, the voltammogram shows the effect of the increase of the upper potential limit. The oxidation peak above 0.9 V corresponds to the oxidation of As(II1) to As(V). This current is due mainly to oxidation of the arsenic in solution. Nevertheless, it has been remarked in Fig. 3a that at this potential, irreversibly adsorbed As(II1) was also oxidized and desorbed. An important point in the case of the polarization limits chosen in Fig. 5b is that, despite the oxidation of As(II1) to As(V), arsenic is always present on the platinum surface. From the window-opening effect at the lower potential limit (Fig. 5c) on the height of the reduction peak at 0.46 V it may be stated that the reduction current below 0.3 V corresponds to the deposition of arsenic. Thus, the peak at 0.46 V corresponds to the oxidation of As(O) to As(II1) as a soluble compound, while the As(II1) to As(V) reaction takes place above 0.9 V. From the measurement of the electric charge, it is clear that a multilayer of arsenic may be deposited in this way. Thus, for the system under consideration, the conditions for normal UPD do not allow the observation of a submonolayer or
200
a
VpA.cm-’ 460.
b
240.
0
0.1
Fig. 5. (a) The first three vokammetric sweeps between 0.3 and 0.8 V of flame-cleaned Pt (111) in 0.5 M H,SO, + 1O-3 M As(II1). (b) () Stationary voltammogram between 0.3 and 0.86 V after (a). (__) Oxidation of As(U) to As(V) above 0.9 V. (c) Window-opening experiment showing bulk arsenic deposition and inhibition of the hydrogen evolution reaction.
monolayer, but only thick film deposition. None of the voltammograms reported in Fig. 5 presents the characteristic reversible surface redox reaction observed with irreversibly adsorbed arsenic.
201
(111.2.2) Arsenic deposition from 10 - 5 M As(III) solution It is clear that the irreversible adsorption of arsenic is an important complication in the classical UPD concept when the adsorbable species is electrodeposited in equilibrium from a solution of its ions at a given concentration.
Fig. 6. (a) Evolution of the voltammogram profile of a flame-cleaned Pt (111) surface in contact with 0.5 M H,SO, + 10s5 M As(II1). Initial potential: 0.75 V. First (- . -), fifth ( -), tenth (- - -) and 20th (- - -) cycles showing the progressive disappearance of the surface redox reaction. (b) Effect of the ) Stationary profile after (a) with E, = 0.7 or 0.8 V; increase of the upper potential Iimit E,. ((- . -) Eu = 1.0 V, (- - -) EU = 1.05 V showing the progressive appearance of the surface redox reaction.
202
In order to understand better how the irreversible adsorption process is masked and mixed with the deposition experiment, it is necessary to form in a more progressive manner the multilayered surface compound. This may be achieved by working with a more dilute As(II1) solution and by recording the effect of the accumulation of arsenic at the surface on the redox surface reaction of irreversibly adsorbed arsenic. It is also interesting to consider the reverse effect of dissolution of an excess of accumulated arsenic on this redox reaction using various procedures. Therefore, successive voltammetric cycles were recorded in a lop5 M As(II1) after flame treatment of the electrode by choosing the upper potential limit to avoid oxidation of As(II1) to As(V). The voltammetric profile changes continuously, including the number of peaks (Fig. 6a). Noteworthy is the appearance of a reduction peak at 0.18 V which becomes more and more marked upon cycling. This peak corresponds to the bulk deposition of arsenic, as has been shown from deposition with higher arsenic concentrations. The electric charge for the different voltammograms increases with the number of cycles until a constant value of nearly 270 &/cm’ is obtained irrespective of the voltammogram profile. However, the charge transferred in the potential range corresponding to the redox process of the irreversibly adsorbed arsenic first increases with the cycling to a maximum value and then vanishes. It can be shown that arsenic dissolution occurs only when the As(III)-As(V) reaction takes place. This is achieved by putting the upper potential limit above the potential of the As(III)-As(V) reaction. Then, after some cycles up to 1 V, the arsenic accumulated in the previous experiment is partially dissolved and the inverse transformation to that reported in Fig. 6a may be observed. The pair of peaks in the potential region around 0.55 V appears progressively when arsenic is dissolved (Fig. 6b). These experiments show that (i) the excess of arsenic masks the redox process of the irreversibly adsorbed adatom; (ii) the amount of arsenic on the surface also depends on the upper potential limit; and (iii) As(V) is formed from the deposited arsenic which is then dissolved as in the irreversible adsorption experiments. These facts imply that the arsenic coverage cannot be determined in the classical UPD concept. However, the evolution of the voltammetric profile suggests a criterion for the detection of the building of the second layer, i.e. when the redox surface reaction of irreversibly adsorbed arsenic loses its sharp voltammetric profile. (III.2.3) Hydrogen reaction and arsenic deposition In arsenic deposition experiments from a 1O-3 M solution, in contrast to experiments with irreversibly adsorbed arsenic, the hydrogen evolution reaction is considerably inhibited: the reaction takes place at negative potential values versus a RHE on the surface covered by thick arsenic films (Fig. 5~). So, this experiment shows that hydrogen evolution on multi-layered (bulk) arsenic is highly inhibited relative to pure Pt (111). An attempt at the oxidation of dissolved molecular hydrogen was unsuccessful in the 10e3 M arsenic solution (Fig. 5b) at a potential of 0.3 V on a Pt (111) surface cycled several times from 0.3 to 1.1 V avoiding bulk arsenic deposition. The absence
203
of an oxidation current implies complete modification of the hydrogen dissociation properties of the surface. This fact also agrees with the previous assumption that under classical UPD conditions the amount of adsorbed arsenic reaches values higher than that required for complete blocking of the hydrogen adsorption sites. This result provides evidence for the origin of the impossibility of control of the surface layer composition in the submonolayer range with this type of deposition experiment in contrast to irreversible adsorption, and shows clearly the formation of at least a compact arsenic layer before the deposition at 0.3 V takes place forming a multilayered arsenic film. (111.2.4) Stability of arsenic after classical UPD conditions The stability of the arsenic multilayer can be tested by the following experiment. An arsenic multilayered Pt (111) electrode is taken out of the arsenic solution at 0.4 V, rinsed and introduced into the test solution containing hydrogen. The appearance of a hydrogen oxidation current (Fig. 7) and the observation of a redox process as in the case of the highest coverages in the irreversible adsorption experiments mean that, with respect to the arsenic amount reported in Fig. 5, some arsenic has been dissolved from the surface. However, the surface remains completely blocked for hydrogen adsorption. This experiment contrasts with the irreversible adsorption results where the arsenic adlayer was found to be stable under the same experimental conditions. It may be concluded that it is this part of the irreversibly adsorbed arsenic which stays on the surface after the transfer of the arsenic multilayered electrode. These two different behaviours of the arsenic adlayer indicate that two kinds of arsenic adatoms exist: a stable adlayer of irreversibly adsorbed arsenic on Pt (111) sites, and arsenic atoms adsorbed in excess forming agregates or multilayers. The
Fig. 7. Voltammogram of a Pt (lll>modified electrode obtained after arsenic deposition from 10W3 M As(III), rinsed and transferred into 0.5 M H,SO, after deaeration by argon bubbling. (1) shows the blocking of the hydrogen adsorption; (2) shows the arsenic surface redox reaction; (3) shows molecular hydrogen oxidation in the same solution after hydrogen bubbling.
204
dissolution of the latter could be due to oxidation by oxygen dissolved in the water used for rinsing. (111.3) Hydrogen evolution on Pt (I II) electrodes precovered arsenic
with irreversibly adsorbed
Arsenic-precovered Pt (111) electrodes were prepared at different coverages ranging from 0 to 0.45 as described in Section (111.1). As has been pointed out, hydrogen evolution in the absence of dissolved hydrogen takes place in the same potential range at a surface blocked or not for hydrogen adsorption by irreversibly adsorbed arsenic. So, the Tafel slopes under such conditions for an arsenic-precovered Pt (111) electrode as well as for a pure Pt (111) surface were determined. This contrasts with the case of UPD experiments where the hydrogen evolution reaction was clearly inhibited as a consequence of the forced adsorption of arsenic. The Tafel plots give a constant slope value of 30 mV (Fig. 8) for all the arsenic coverages, ranging from 0 up to the completion of the stable adlayer which completely blocks hydrogen adsorption (6”, = l/3). With higher surface amounts of irreversibly adsorbed arsenic, 6,, > l/3, the reaction slows down at high current densities and the Tafel slope is 30 mV at low current densities only. It has been verified that the slope was independent of the sweep rate in a range including that used in Fig. 8, i.e. from 5 to 50 mV/s.
Fig. 8. Hydrogen evolution on an As-modified Pt (111) electrode from 0.5 M H,SO, hydrogen-free solution. Tafel plots at different arsenic coverages: 8,, = 0 (0); 0.15 (X); 0.22 (A); 0.4 (0); 0.43 (V). 10 mV/s.
205
i,A lo+
cm“
y
Fig. 9. Current density measured at 0 V for the hydrogen evolution reaction vs. wsenic coverage on the Pt (111) surface.
Figure 9 shows the decrease of the current density of the hydrogen evolution reaction at 0 V as a function of the arsenic coverage. This graph has two sections. There is a clear change in the rate of the current density decrease with arsenic coverage when complete inhibition of the hydrogen adsorption monolayer is reached (e,, = l/3). At lower coverages, the current densities decrease slowly, but at higher coverages there is a sudden fall in the current together with an increase in the Tafel slope, as deduced from Fig. 8. It is important to remark that some experiments carried out with the same technique lead to current density values lower than those reported in Fig. 9. Two such points have been plotted in Fig. 9 as open circles. In the corresponding cases, the sum QAs + Qu was always found to be lower than 240 @/cm* (around 220 @Z/cm*). The deviation in the electrical charge indicates that an occasional surface contamination occurred in the course of the experiment. This conclusion agrees with the low current density found. These points can be considered as irrelevant but the following conclusions may be emphasized: (a) in order to check the surface cleanliness, both the hydrogen and the adatom electric charges must be measured with their sum equal to 240 PC/cm*; (b) the contamination affects mainly the determination of the hydrogen coverage with two consequences: an increase in the number of electrons per site for the surface redox reaction, yielding an apparent value, and a decrease in the effective hydrogen evolution current. So, the presence of arsenic on the Pt (111) surface does not induce any dramatic change in the kinetics of the hydrogen evolution reaction until 0,, = l/3. It is interesting to note that the current density at 6,, = l/3 is two-thirds that measured at 0,, = 0. This may be correlated with the stoichiometry of one arsenic atom per three surface platinum atoms found at this particular coverage. This result is
206
unexpected because the current does not fall with a quadratic (1 - 8,) as in other cases [2,8].
dependence
on
(IV) DISCUSSION AND CONCLUSION
The main results of this work may be summarized as follows. The decrease in the adsorption capability of platinum down to zero with increasing amounts of adsorbed arsenic up to 0,, = l/3 is accompanied by a decrease in the current density of hydrogen evolution which is nearly proportional to the arsenic coverage with a constant Tafel slope of 30 mV. In the same range of arsenic coverage, molecular hydrogen may be oxidized too. Higher arsenic coverages may be reached; then the hydrogen evolution reaction rate decreases more rapidly and the hydrogen reaction is blocked when the arsenic multilayer builds up. The effect of arsenic adsorption at a,, = l/3 on the hydrogen evolution rate when hydrogen adsorption is zero indicates that the underpotentially (UPD) adsorbed hydrogen can be considered as a species independent of the mechanism of hydrogen evolution, This conclusion was also reached by other authors using other experimental approaches [l-3]. This characteristic coverage implies that platinum atoms are not available for UPD adsorbed hydrogen, but are available for hydrogen recombination or hydrogen dissociation, the latter implying a hydrogen adsorption step with an adsorbed hydrogen intermediate which cannot be of the UPD type. These apparently conflicting results may be reconciled by considering the position of the arsenic adatoms with respect to that of the substrate atoms. Let us consider the two possible positions an arsenic adatom may occupy on a Pt (111) surface which may be hardly distinguished by the LEED technique. The first one is the hollow position (octahedral site). In this case, an arsenic atom has three platinum atoms in the position of first nearest neighbours as shown in Fig. 10a. When the (fi X &)R 30” adlayer is completed, all the platinum atoms are bonded to an arsenic atom and are homogeneously influenced by the presence of arsenic adatoms. The second position to be considered is the on-top position. In this case, presented in Fig. lob, every arsenic adatom has only one platinum atom in the position of first nearest neighbour. When the structure (6 x 6) R 30” is completed, only one-third of the surface platinum atoms are bonded to an arsenic atom; the other two-thirds correspond to platinum atoms with orbitals free for adsorption bonding. With the on-top model, it can be seen that the octahedral sites have one of the three platinum surface atoms bonded to an arsenic atom. This breakdown of the site symmetry may explain the blocking of hydrogen adsorption on the site. At 0,, = l/3, all the octahedral sites are thus affected by the presence of arsenic adatoms and hydrogen adsorption could be completely inhibited. In this surface structure, the platinum atoms which are not in the position of first nearest neighbour are less affected by the presence of adsorbed arsenic and their three-fold symmetry is preserved. They could be the platinum atoms which participate in the hydrogen recombination or molecular hydrogen dissociation in the hydrogen reac-
a
b
Fig. 10. Hard sphere model for the (& X 6) R 30° structure of an overlayer of heteroatoms (A) on a (111) foe substrate. (a) A in hollow position. (b) A in on-top position. (1) refers to the substrate atoms in the position of fist nearest neighbours relative to A. (2) refers to the second nearest neighbour. (0) position of the octahedral sites around A.
tion. This model could explain the decrease in the hydrogen evolution current (Fig. 9), when the (6 x 6) R 30” overlayer is forming, down to a value which is two-thirds of its initial value at completion of the overlayer. The change of slope above 6,, = l/3 shows that the adatoms adsorbed at a higher coverage are more efficient for the inhibition of the hydrogen evolution reaction while they cannot have any additional effect on hydrogen adsorption which is fully blocked. The effect on the hydrogen evolution reaction may be tentatively explained by assuming that an arsenic atom adsorbing at such a coverage, in addition to its own inhibiting effect, increases the inhibiting effect of the arsenic atoms already adsorbed. A possibility would be that the latter increase their number of first nearest neighbours. In contrast to the previous conclusions drawn for the on-top model, the hollow model suggests that both the UPD hydrogen blocking and the inhibition of the hydrogen evolution reaction could change to the same extent with arsenic coverage, If it can be definitely proved that a surface platinum atom in the position of first nearest neighbour with an arsenic adatom is inactive for hydrogen recombination or dissociation, then the hollow model can be excluded in accounting for the present results and the on-top model works. in the latter, evidently the hydrogen intermediate, which is an adsorbed species, implies that platinum atoms give rise to a type of adsorption site different from that used for UPD adsorbed hydrogen. This conclusion agrees with that in ref. 3. This remains an open question for which further work is required. This may be considered as a qualitative approach which tries to reconcile the hydrogen adsorption properties of platinum and the well-known hydrogen reaction in which a type of adsorbed hydrogen different from the UPD one is involved as an intermediate. In the case of well-defined platinum electrodes
208
such as Pt (ill), the microscopic aspect of the problem can be better defined and there is now a sufficient amount of data with Pt (ill)-adatom systems to think that the observations reported in this paper and the attempt at explaining the behaviour of the system may be generalized to other irreversible adsorbed adatoms using both UPD adsorbed hydrogen and the hydrogen reaction as surface probes. ACKNOWLEDGEMENTS
We are grateful to Dr. D. Aberdam for helpful ~~ussions. One of us (J.C.) ackuowledges DGYCIT de1 Ministerio de Education y Ciencia (Spain) and Direction de la Cooperation Scientifique et Technique du Ministere de Affaires Etrangeres (France) for a 3-month stay at the Department of Physical Chemistry of the University of Alicante as part of the Mercure programme. The financial support of the DGICYT through project No. PBS7-0795 is also gratefully acknowledged. REFE~NCXS 1 2 3 4 5 6 7 8 9 10 11 12 13
B.E. Conway, Sci. Frog. Oxf., 71 (1987) 479. E. pllotopopoff and P. Marcus, J. Vat. Sci. Technol., 5 (1987) 944. R.J. Nichols and A. Bewick, J. Electroam& Chem., 243 (1988) 445. J. Clavilier, R. Faure, G. Guinet and R. Durand, J. EIectroanai. Cbem., 107 (1980) 205. J. Clavilier, J. Etectroanal. Chem., 107 (1980) 211. J.M. Feliu, A. Fernandez-Vega, A. Aldaz and J. Clavilier, J. Electroanal. Chem., 256 (1988) 149. J. Clavilier, J.M. Feliu and A. Aldaz, J. Electroanai. Cbem., 243 (1988) 419. RR. Adz& in H. Gerischer (Ed.), Advances in Ekxtrocbemistry and EiectrochemicaI Engineezing, Vol. 13, Wiley, New York, 1984, p. 159. N. Furuya and S. Motoo, J. Electroanal. Chem., 78 (1977) 243. G.A. Somotjai, J. Chem. Phys., 54 (1971) 389. M.T. Paffett, CT. Campbell and T.N. Taylor, J. Chem. phys., 85 (1986) 6176. N. Furuya and S. Motoo, J. E%x~roanai.Chem., 98 (1979) 189. T. Loucka, J. EIectroanaI. Chem., 47 (1973) 103.