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Heterogeneous electrocatalysis on well-defined platinum surfaces modified by controlled amounts of irreversibly adsorbed adatoms
229
J. Electroanal. Chem., 305 (1991) 229-240 Elsevier Sequoia S.A., Lausanne
Heterogeneous electrocatalysis on well-defined platinum surfaces modified by controlled amounts of irreversibly adsorbed adatoms Part IV. Formic acid oxidation on the Pt( 111) -As system A. Fernandez-Vega, Departamento
J.M. Feliu and A. Aldaz
de Quimica-Fisica,
Universidad de Alicante, Apartado 99, 03080 Alicante (Spain)
J. Clavilier Laboratoire (France) (Received
d’Electrochimie
Interfacinle
du C.N.R.S.,
9 July 1990; in revised form 1 October
I Place Aristide
Briand,
92195 Meudon
Cedex
1990)
Abstract
The effect of arsenic irreversibly adsorbed on Pt(ll1) has been studied for the electrocatalytic oxidation of formic acid in the whole range of coverage. The maximum intrinsic oxidation rate on Pt(ll1) has been measured using pulsed voltammetry. The presence of adatoms leads to an increase in the activity for the direct reaction by a factor 1.5 in the range of potential where the adatoms are in their reduced state. When their oxidation takes place, the current of formic acid oxidation falls exactly in the potential range where the adatom oxidation occurs. Once the adatoms are in their oxidized state, the activity of the catalyst is lower than that of pure Pt(ll1). The presence of adatoms influences the selectivity inhibiting the poison formation even at low arsenic coverage suggesting an electronic effect at the surface induced by As adatoms. A model with two independent reactions taking place on two active surface domains (the Pt free sites and the As covered ones) for the active intermediate pathway has been proposed to account for the effect of adatoms in the potential range where they are in their reduced state.
INTRODUCTION
Formic acid electrooxidation on platinum is a classical model for a structure sensitive reaction, both for the direct pathway, via an active intermediate, and for a 0022-0728/91/$03.50
0 1991 - Elsevier Sequoia
S.A.
230
parallel pathway which leads to the poisoning intermediate for the surface blocking of the electrocatalyst [l].
HCOOH/Pt
Active surface intermediate
-
Poisoning intermediate
-
El
formation responsible
CO,
+ H,O
7
h
E2
co
2
where E2 > E,. From a kinetic point of view, the Pt(ll1) electrode surface has been shown to possess a surface structure of low activity for both pathways [2,3]. It is known that the presence of various adatoms on the platinum electrode surfaces increases the formic acid oxidation rate on polycrystalline platinum [4-61 or platinum single crystals [7]. In the case of irreversibly absorbed bismuth on Pt(ll1) an increased rate has been found for the active intermediate pathway while the poisoning intermediate is not detectable even at very low bismuth coverages [8]. This latter result contrasts with that observed for the (100) orientation where the poison is still observed for Bi coverages below 0.7 measured with respect to hydrogen adsorption (see below) [9]. Arsenic can be adsorbed irreversibly on the Pt(ll1) electrode giving rise to a surface redox process which leads to the formation of an oxygenated species in a narrow potential range. For that, it is easy to determine the oxidation state and the surface composition for different arsenic coverages [lo]. The aim of this work was to study the influence of arsenic on the classical poisoning of platinum in the formic acid oxidation reaction. In pathway 2 special attention was paid to the possibility of either an inhibition of CO formation or an inhibition of the CO adsorption step leading to its accumulation on the surface. This point was checked by studying the behaviour of the Pt(lll)-As electrode after CO exposure. Intrinsic activity of Pt(ll1) for formic acid oxidation was measured in the absence of poisoning intermediate on the surface in the whole range of potential by using pulsed voltammetry [ll]. In an attempt to increase the low activity of Pt(ll1) the possible catalytic effects of arsenic adatoms on the active intermediate reaction path has also been studied. EXPERIMENTAL
The chemicals, electrodes, apparatus and cells used have been described elsewhere [9,10]. The test electrolyte was 0.5 M H,SO, used for checking the hydrogen adsorption and for studying the surface redox reaction of arsenic adspecies. Arsenic modified Pt(ll1) electrodes were prepared by immersion of the single crystal sample in lop3 M solution of As,O, (Merck p.a.) in 0.5 M sulphuric acid solution.
231
For the study of formic acid oxidation a 0.25 M formic acid (Merck p.a.) in 0.5 M sulphuric acid solution was employed. Therefore two cells for experiments were required. The first cell containing the test electrolyte was used to characterize the bimetallic surface. A second cell containing the formic acid solution was used to check the electrocatalytic activity of the surface. The poisoning by formic acid was carried out by the technique described in refs. 12 and 13. The dissociative adsorption experiments were performed under an argon atmosphere in 0.1 M HCOOH + 0.5 M H,SO,. The adsorption and accumulation step in the poisoning process was investigated by exposure of Pt(lll)-As samples to a CO saturated 0.5 M H,SO, solution [14]. Pulsed voltammetric experiments were carried out following the same technique as described previously [ll]. Potentials were measured against a reversible hydrogen electrode (RHE). RESULTS
Pure Pt(l1 I) surfaces The stationary voltammogram for the formic acid oxidation on pure Pt(ll1) electrode shows a plateau in the positive sweep (Fig. 1 solid line). At the positive limit of the plateau a small peak at 0.72 V is visible relative to the surface oxidation and elimination of the adsorbed poison [12]. As a consequence of this oxidation, the negative-going sweep yields current densities higher than those obtained in the positive-going one. The difference between both current densities is not very large so the poisoning rate seems to be slow. In order to determine the intrinsic activity of the Pt(ll1) electrode for formic acid oxidation through reaction path 1 an experiment of pulsed voltammetry was carried out. With this technique [ll], the poison is oxidized and eliminated by potential pulses which maintain the electrode surface structure unaltered during the recording of the voltammetric current which is sampled after every pulse. The
I
I
#
I
I
0.5
I
I
I
I
E/V
(rhe)
Fig. 1. Stationary voltammogram of HCOOH oxidation on Pt(lll) in 0.5 M H,SO, voltammogram (-). Pulse voltammetry conditions: sampling window time: 0.38 s; pulse duration: 0.7 s. Potential of the pulse: 0.9 V; time interval between the end of the pulse and opening of the sampling window: 0.03 s. Sweep rate: 20 mV s-‘.
pulsed ( -), poison oxidation poison oxidation
232
corresponding voltammogram presents only a low increase of the current density (Fig. 1 dashed line) as compared with the conventional voltammetric sweep (solid line). In the latter case poison accumulates slowly on the surface for both directions of sweeping except above 0.72 V where the pulse voltammogram merges with the usual one, independent of the sweep direction for the latter. The current pulse profiles suggest that the deactivation process of the Pt(ll1) surface is slow. A comparison with the Pt(lOO) surface for the same experimental conditions [ll], where a fast decrease of the current pulse is observed, reinforces this conclusion. This result agrees with previous observations in which the Pt(ll1) surface presents the maximum ‘degree of resistance against poisoning’ [2]. On the other hand, this experiment also indicates the low intrinsic activity of the (111) orientation, compared to Pt(lOO), for formic acid oxidation via the active intermediate. Thus, the previous assumption [2] is confirmed that the Pt(ll1) orientation which presents the most densely packed atomic arrangement of all platinum faces is characterized by a low activity for the two reaction paths in formic acid oxidation. Behaviour of As precovered Pt(li1) electrodes for formic acid oxidation As has been shown [lo], arsenic can be adsorbed irreversibly on Pt(ll1). The redox process undergone by the adatom gives rise to reversible voltammetric oxidation-reduction peaks. The adsorbate valence change with formation of an arsenic oxygenated adspecies occurs in a narrow potential range. The potential range where the voltammogram of the Pt(lll)-As electrode is usually recorded, 0.06-0.75 V, is a potential range which corresponds to the stability domain of the adatom on the Pt(ll1) oriented electrode. The maximum arsenic coverage relative to the platinum surface atoms of the substrate is l/3 as shown in ref. 15. Because each arsenic atom blocks three hydrogen sites such a coverage gives a complete blocking of hydrogen adsorption. In other words, this means that if coverage is defined relative to the blocked hydrogen sites the alternative maximum value could be 1. The latter definition was that used in ref. 10. In order to distinguish clearly between the two possible definitions of the coverage (0) by an adatom (here As), O,, or O,,(H) will be used, if the coverage refers to platinum surface atoms or to adsorbed hydrogen, respectively. In the region of low arsenic coverages, the voltammogram (Fig. 2a) shows clearly the reversible redox surface process of the adatom with a peak potential at 0.58 V and hydrogen adsorption on the surface platinum sites free of adatoms. Typical features of hydrogen adsorption-desorption on the partially blocked Pt(ll1) sites are clearly visible, mainly regarding the distribution between the unusual hydrogen adsorption state, which is shifted positively with increasing amounts of adatoms [13], and the weakly bonded hydrogen state. In the same way, at sufficiently low arsenic coverages, the rest of the sharp peaks corresponding to the surface phase transition of adsorbed anions can be observed, and the related adsorption hydrogen isotherm change [16] which is characteristic of this platinum orientation in sulphuric acid solutions. Figure 2b shows the catalytic effect of adsorbed arsenic on the formic acid
233
I /pA
cm**
(4
I
IO -
i/mA
0.1
M
cm-*
E/V
(rhe)
Fig. 2. (a) (- - -) Voltammogram of the Pt(ll1) surface. () Voltammogram of the surface modified after the irreversible adsorption of arsenic /I,, = 0.11. Test solution: 0.5 M H,SO,; o = 50 mV s-‘. (b) () first voltammogram of HCOOH oxidation obtained with the same modified electrode as in (a), in a 0.25 M HCOOH+0.5 M H,SO, solution. (---) Stationary voltammogram of HCOOH oxidation on Pt(ll1) between the same potential limits.
oxidation reaction with the modified electrode giving the voltammogram (solid line) reported in Fig. 2a. In this figure a comparison of the voltammogram of the modified electrode with that yielded by pure Pt(ll1) (Fig. 2b dashed line) shows a current increase in the potential range below 0.6 V. It is interesting to observe that there is an increase in the activity in the potential range where the adatoms are in their reduced form and that the current suddenly falls when adatom oxidation takes place. The sequence is reversed with the direction of sweeping. The sudden reversible change of activity between 0.54 and 0.6 V is closely correlated with the reversible change of the valency state of the adatom observed in Fig. 2a. In the potential region where heteroatoms are in the oxidized state, the current reaches values always lower than those obtained on pure Pt(ll1) and independent of the direction of sweeping. It should be pointed out that the negativeand positive-going sweeps in the whole range of potential practically coincide. The
234
I /pA
(4
cmd2
\
1 I/mA
cm-*
M
5t
3-
l_
O-
, 0.1
0.5
E/V
(rhe)
Fig. 3. (a) Voltammogram of a Pt(lll)-As modified electrode at high coverage O,, = 0.31 in the test solution 0.5 M H,SO,; v = 50 mV s-‘. (b) First voltammetric cycle obtained with the same modified electrode as in (a) in 0.25 M HCOOH+0.5 M H,SO,; v = 50 mV s-l.
voltammogram is very stable with cycling. Holding the voltammetric sweep at the potential of maximum current maintains a nearly constant oxidation rate for formic acid. Moreover, it is possible to observe, for some coverages, that the current even increases slightly with the holding time. The redox peaks of the irreversibly adsorbed arsenic split when arsenic coverage approaches a monolayer (Fig. 3a) on Pt(ll1). The hydrogen charge decreases
235
proportionally, as studied in ref. 15, confined in the zone below 0.2 V, while the unusually bound hydrogen state vanishes. The formic acid oxidation reaction on these highly covered surfaces yields greater oxidation peak currents than described for lower arsenic coverages. However, the beginning of formic acid oxidation is shifted to more positive potential values. As a consequence of this the ascendent branch becomes concave and the oxidation peak narrows progressively with coverage (Fig. 3b). The potential of the current maximum remains constant and coincides with the valence state change of the adatom. The oxidation of formic acid at the lower potential range is inhibited slightly by increasing amounts of adsorbed arsenic. As in the case of low arsenic coverage, this voltammogram shows practically no hysteresis and the small effect observed between 0.54 and 0.60 V is easily explained in terms of the slight irreversibility in the surface redox reaction of the adatom. It should be pointed out also that the voltammetric current is quite stable with cycling. During arrests of the sweep in the ascendent and descendent branches of the voltammogram the current was constant. Slight current decreases were observed with arrests at the potential of the voltammogram apex while at lower coverages a slight increase has already been mentioned. Inhibition of poison intermediate reaction path Attempts at isolating the surface poison formed by dissociative adsorption [12,13] have failed even at low arsenic coverages. Figure 4 shows the nearly unchanged voltammetric profile after trying as usual to form the poison on an arsenic electrode of low coverage using a 0.25 M HCOOH + 0.5M H,SO, solution. No significant variation of the amount of hydrogen adsorbed on the As modified electrode could be detected either after exposure to formic acid or after a cycle in a region where the poison could be desorbed from the surface. On the other hand, no oxidation current in the potential range of the poison oxidation was detected. This result is surprising in view of the large number of hydrogen free sites or unblocked surface platinum atoms. The voltammetric profile suggests the existence of large Pt(ll1) domains on the partially arsenic covered electrode more than 12 platinum atom diameters wide in their lower dimension [17]. The impossibility of detecting the poisoning species on As precovered Pt(ll1) electrodes for O,, > 0.09 (minimum coverage studied in this work) shows that there is a large range of coverage for which pathway 2 does not affect the reaction pathway 1. This observation is similar to that obtained with irreversibly adsorbed bismuth on Pt(ll1) [8], where it was not possible to detect the presence of the poisoning intermediate from dissociative adsorption experiments performed under the same conditions as in the case of arsenic. This absence of poison on the surface could be inferred from the coincidence between the negative and positive sweeps in the voltammograms of the electrocatalytic oxidation of the formic acid at different arsenic coverages (Figs. 2b and 3b). The loss of the efficiency of pathway 2 for the blocking of reaction pathway 1 in the presence of arsenic on the Pt(ll1) surface deserves a special examination. It would be interesting to know how the adsorbed adatoms affect the inhibition of
236
a
Fig. 4. Negative voltammetric test for poison accumulation on the Pt(ll1) electrode at low arsenic coverage O,, = 0.09. (a) (- - -) Before poison formation; (b) (-) after exposure to the HCOOH solution, with different upper potential limits. Dissociative adsorption experiment attempted from 0.1 M formic acid in 0.5 M H,SO,; u = 50 mV SK’. Test solution 0.5 M H,SO,, inmersion potential 0.2 V.
pathway 2. The first possibility would be that poison cannot be formed by formic acid dissociation on the As-Pt(ll1) modified surface. A second possibility would be that poison cannot be accumulated on the modified surface after its formation assuming that dissociative adsorption of formic acid yields CO,, directly. In an attempt to clarify this characteristic behaviour of the As-Pt(ll1) modified electrode some experiments have been carried out to check whether CO could be adsorbed from a 0.5 M H,SO, solution using the same routine experiment as that described with formic acid solutions. The positive test for the detection of the adsorbed CO on a partially arsenic covered Pt(ll1) electrode is presented in Fig. 5. In this experiment almost complete blocking of the hydrogen adsorption sites is achieved after the exposure of the electrode to the CO solution (curve a). In the subsequent negative-going sweep the adsorption of hydrogen ability of the modified surface is restored (curve b). It should be pointed out that the Pt(lll)-As electrode used was initially covered more than the one used in the dissociative adsorption experiment (Fig. 4). Comparison of curves a and b in Fig. 5 shows that in cycle b the arsenic surface redox
237
Fig. 5. Positive voltammetric test for adsorbed CO detection from a CO saturated 0.5 M H,SO, on Pt(lll)-As electrode (As = 0.11). (a) After CO adsorption; (b) after CO oxidation-desorption.
solution
reaction is restored as before CO adsorption. This means that (i) hydrogen adsorption at full coverage of the arsenic free sites has played a role in the restoration process (ii) the oxidation of adsorbed arsenic occurs at lower potentials in the absence of adsorbed CO than when the surface is saturated indicating that adsorbed CO protects AS,~ against oxidation. On the other hand, after CO oxidation the amount of arsenic on the surface is slightly decreased because the upper potential limit required for complete CO oxidation is just outside of the potential range of stability of Asad. As a consequence, from the CO adsorption experiment it seems evident that the adatom acts on the Pt(ll1) surface through the inhibition of the poison formation reaction path because after its formation the poison can be accumulated on the surface. Moreover, the result of the experiment of CO adsorption has been confirmed even in the presence of 1 M formic acid in the CO containing solution. Apex voltammetric current densities and current densities obtained at 0.35, 0.45 and 0.5 V have been plotted versus the arsenic coverage (Fig. 6). It should be noted that the maximum current measured at 0.54 V increases progressively, showing a linear dependence on the surface arsenic concentration. This result points out that the surface completely blocked for hydrogen adsorption by arsenic can oxidize formic acid at the highest rate, nearly 1.5 times that corresponding to the intrinsic activity value of Pt(lll), obtained by pulse voltammetry. Nevertheless, currents measured at lower potentials show different optimum surface adspecies concentrations. In this way, current densities increase first with 8,, until a maximum current density is attained. From these maximum values a higher heteroatom coverage leads to a progressive inhibition of the ascendent branch of the voltammetric peak of formic acid oxidation reaction. The position of this maximum in the I-O,, curve is potential dependent: at 0.35 V the optimum coverage is obtained at @,, = 0.1-0.16 while at 0.5 V it is obtained around 0.27. This shows a progressive inhibition with
238
Fig. 6. (0) Plot of maximum current densities vs. arsenic coverage measured in the first voltammetric sweep of formic acid oxidation at 0.54 V. Current density vs. arsenic coverage at different potentials: (v) 0.35, (0) 0.45 and (A) 0.50 V. (- - -) Position of intrinsic activity of Pt(ll1) at @,, = 0.
increasing
arsenic
coverages
of
the
formic
acid
oxidation
current
in the low
potential range. DISCUSSION
The effect of adsorbed arsenic on the poison formation even at a low coverage similar to that of bismuth, suggests that these adatoms have a large electronic effect on the electrocatalytic properties of Pt(ll1). In the case of bismuth a large decrease of the work function with bismuth coverage has been reported [18]. A similar effect may be expected with As adatoms in view of the large inhibiting effect on the dissociative adsorption reaction. This means that surface platinum atoms of Pt(ll1) free of arsenic may have their catalytic activity modified considerably by the presence of small amounts of arsenic on the surface, as regards poison formation. The results reported in Fig. 6 in the absence of poisoning intermediate are thus relative to the direct oxidation pathway. They can be explained through the existence of two types of independent reactions for the formic acid oxidation observed predominantly at the beginning of oxidation and near the maximum of the voltammetric current, respectively. The first one would take place in the low potential zone and have its optimum arsenic coverage at low arsenic coverages. For this reason it may be assumed that the reaction occurs on the surface domains free of arsenic, which represent the major part of the surface. The second one would proceed at higher potentials and its rate increases with increasing arsenic coverages. This indicates that the reaction may
239
takes place on the surface domains where arsenic is adsorbed. This is in agreement with the fact that at maximum arsenic coverages the current density is also at a maximum. All the values reported in Fig. 6 are relative to surface conditions where the adatoms are in their reduced state (zero valency state). The combination of both contributions along the ascendent branch of the voltammogram may explain the variation of the optimum coverage value in the lower potential range as reported above and the linear activity increase of the electrode with arsenic coverage in the peak potential region. The maximum of activity for formic acid oxidation is observed when arsenic is in its reduced form below 0.54 V; the activity at all arsenic coverages falls when arsenic is present on the surface in a combination with oxygen, and becomes lower than that obtained on pure Pt(ll1) electrodes. The chemical change due to surface arsenic oxidation decreases the catalytic properties of the overall surface sample considerably and probably induces an inhibiting effect on those platinum surface atoms that are combined with the oxygenated arsenic surface compound.
CONCLUSIONS
Experiments carried out on CO adsorption from a CO saturated solution show that the arsenic present on Pt(ll1) surfaces inhibits the poison formation reaction path in formic acid oxidation and that it does not block the adsorption of the reaction product. This phenomenon lies at the origin of the negative results obtained for isolation of the poison formed from formic acid dissociative adsorption experiments on such modified surfaces. Nevertheless, when CO has been adsorbed from the CO saturated solution the oxidation of CO gives rise to a complex voltammetric profile in the potential range where As is combined with oxygen. CO,, appears to protect adsorbed As against oxidation. It cannot be excluded that this compound participates in the incipient oxidation of adsorbed CO by providing oxygen atoms necessary for CO, formation. This result is similar to that observed for bismuth irreversible adsorption on the Pt(ll1) surface [8] and it suggests a considerable change in the electrocatalytic properties of the As modified surface which is significantly more selective and active for the reaction pathway via the active intermediate. The maximum activity at O,, = l/3 was found to be 1.5 times higher than the intrinsic activity measured at O,, = 0. In both cases the potential is the same (0.54 V). The analysis of the formic acid oxidation current at different potentials and coverages suggests the presence of two types of reaction taking place: on the one hand at the arsenic free platinum surface domains and on the other hand at those covered by the adatoms. Both reactions combined together give the oxidation current of the overall process. The former would be predominant at the beginning of formic acid oxidation while the latter would be predominant when the oxidation current peak reaches its maximum value. A marked decrease in the activity is
240
observed when AS,,, is oxidized on the surface, forming an oxygenated compound.
surface
ACKNOWLEDGEMENTS
The financial support of the DGICYT through project No. PB87-0795 is gratefully acknowledged. One of us, J.C., is grateful for a stay at the University of Alicante in the framework of the “Mercure” programme for scientific exchange. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
A. Capon and R. Parsons, J. Electroanal. Chem., 44 (1973) 239. S. Motoo and N. Furuya, J. Electroanal. Chem., 1984 (1985) 303. S.G. Sun, J. Clavilier and A. Bewick, J. Electroanal. Chem., 240 (1988) 147. M. Shibata and S. Motoo, J. Electroanal. Chem., 188 (1985) 111. M. Shibata, 0. Takahashi and S. Motoo, J. Electroanal. Chem., 249 (1988) 253. M. Watanabe, M. Horiuchi and S. Motoo, J. Electroanal. Chem., 250 (1988) 117. R.R. Adzic, A.V. Tripkovic and N.M. Markovic, J. Electroanal. Chem., 150 (1983) 79. J. Clavilier, A. Femandez-Vega, J.M. Feliu and A. Aldaz, J. Electroanal. Chem., 258 (1989) 89. J. Clavilier, A. Fernandez-Vega, J.M. Feliu and A. Aldaz, J. Elcctroanal. Chem., 261 (1989) 113. J.M. Feliu, A. Femandez-Vega, A. Aldaz and J. Clavilier, J. Electroanal. Chem., 256 (1988) 149. J. Clavilier, J. Electroanal. Chem., 236 (1987) 87. J. Clavilier and S.G. Sun, J. Electroanal. Chem., 199 (1986) 471. S.G. Sun and J. Clavilier, J. Electroanal. Chem., 236 (1987) 95. J.M. Feliu, J.M. Orts, A. Fernandez-Vega, A. Aldaz and J. Clavilier, J. Electroanal. Chem. 296 (1990) 191. J. Clavilier, J.M. Feliu, A. Femandez-Vega and A. Aldaz, J. Electroanal. Chem., 294 (1990) 193. J. Clavilier, J.P. Ganon and M. Petit, J. Electroanal. Chem., 265 (1989) 231. J. Clavilier, K. El Achi and A. Rodes, Chem. Phys., 141 (1990) 1. M.T. Paffett, CT. Campbell and T.N. Taylor, J. Chem. Phys., 85 (1986) 6176.
J. Electroanal. Chem., 305 (1991) 229-240 Elsevier Sequoia S.A., Lausanne
Heterogeneous electrocatalysis on well-defined platinum surfaces modified by controlled amounts of irreversibly adsorbed adatoms Part IV. Formic acid oxidation on the Pt( 111) -As system A. Fernandez-Vega, Departamento
J.M. Feliu and A. Aldaz
de Quimica-Fisica,
Universidad de Alicante, Apartado 99, 03080 Alicante (Spain)
J. Clavilier Laboratoire (France) (Received
d’Electrochimie
Interfacinle
du C.N.R.S.,
9 July 1990; in revised form 1 October
I Place Aristide
Briand,
92195 Meudon
Cedex
1990)
Abstract
The effect of arsenic irreversibly adsorbed on Pt(ll1) has been studied for the electrocatalytic oxidation of formic acid in the whole range of coverage. The maximum intrinsic oxidation rate on Pt(ll1) has been measured using pulsed voltammetry. The presence of adatoms leads to an increase in the activity for the direct reaction by a factor 1.5 in the range of potential where the adatoms are in their reduced state. When their oxidation takes place, the current of formic acid oxidation falls exactly in the potential range where the adatom oxidation occurs. Once the adatoms are in their oxidized state, the activity of the catalyst is lower than that of pure Pt(ll1). The presence of adatoms influences the selectivity inhibiting the poison formation even at low arsenic coverage suggesting an electronic effect at the surface induced by As adatoms. A model with two independent reactions taking place on two active surface domains (the Pt free sites and the As covered ones) for the active intermediate pathway has been proposed to account for the effect of adatoms in the potential range where they are in their reduced state.
INTRODUCTION
Formic acid electrooxidation on platinum is a classical model for a structure sensitive reaction, both for the direct pathway, via an active intermediate, and for a 0022-0728/91/$03.50
0 1991 - Elsevier Sequoia
S.A.
230
parallel pathway which leads to the poisoning intermediate for the surface blocking of the electrocatalyst [l].
HCOOH/Pt
Active surface intermediate
-
Poisoning intermediate
-
El
formation responsible
CO,
+ H,O
7
h
E2
co
2
where E2 > E,. From a kinetic point of view, the Pt(ll1) electrode surface has been shown to possess a surface structure of low activity for both pathways [2,3]. It is known that the presence of various adatoms on the platinum electrode surfaces increases the formic acid oxidation rate on polycrystalline platinum [4-61 or platinum single crystals [7]. In the case of irreversibly absorbed bismuth on Pt(ll1) an increased rate has been found for the active intermediate pathway while the poisoning intermediate is not detectable even at very low bismuth coverages [8]. This latter result contrasts with that observed for the (100) orientation where the poison is still observed for Bi coverages below 0.7 measured with respect to hydrogen adsorption (see below) [9]. Arsenic can be adsorbed irreversibly on the Pt(ll1) electrode giving rise to a surface redox process which leads to the formation of an oxygenated species in a narrow potential range. For that, it is easy to determine the oxidation state and the surface composition for different arsenic coverages [lo]. The aim of this work was to study the influence of arsenic on the classical poisoning of platinum in the formic acid oxidation reaction. In pathway 2 special attention was paid to the possibility of either an inhibition of CO formation or an inhibition of the CO adsorption step leading to its accumulation on the surface. This point was checked by studying the behaviour of the Pt(lll)-As electrode after CO exposure. Intrinsic activity of Pt(ll1) for formic acid oxidation was measured in the absence of poisoning intermediate on the surface in the whole range of potential by using pulsed voltammetry [ll]. In an attempt to increase the low activity of Pt(ll1) the possible catalytic effects of arsenic adatoms on the active intermediate reaction path has also been studied. EXPERIMENTAL
The chemicals, electrodes, apparatus and cells used have been described elsewhere [9,10]. The test electrolyte was 0.5 M H,SO, used for checking the hydrogen adsorption and for studying the surface redox reaction of arsenic adspecies. Arsenic modified Pt(ll1) electrodes were prepared by immersion of the single crystal sample in lop3 M solution of As,O, (Merck p.a.) in 0.5 M sulphuric acid solution.
231
For the study of formic acid oxidation a 0.25 M formic acid (Merck p.a.) in 0.5 M sulphuric acid solution was employed. Therefore two cells for experiments were required. The first cell containing the test electrolyte was used to characterize the bimetallic surface. A second cell containing the formic acid solution was used to check the electrocatalytic activity of the surface. The poisoning by formic acid was carried out by the technique described in refs. 12 and 13. The dissociative adsorption experiments were performed under an argon atmosphere in 0.1 M HCOOH + 0.5 M H,SO,. The adsorption and accumulation step in the poisoning process was investigated by exposure of Pt(lll)-As samples to a CO saturated 0.5 M H,SO, solution [14]. Pulsed voltammetric experiments were carried out following the same technique as described previously [ll]. Potentials were measured against a reversible hydrogen electrode (RHE). RESULTS
Pure Pt(l1 I) surfaces The stationary voltammogram for the formic acid oxidation on pure Pt(ll1) electrode shows a plateau in the positive sweep (Fig. 1 solid line). At the positive limit of the plateau a small peak at 0.72 V is visible relative to the surface oxidation and elimination of the adsorbed poison [12]. As a consequence of this oxidation, the negative-going sweep yields current densities higher than those obtained in the positive-going one. The difference between both current densities is not very large so the poisoning rate seems to be slow. In order to determine the intrinsic activity of the Pt(ll1) electrode for formic acid oxidation through reaction path 1 an experiment of pulsed voltammetry was carried out. With this technique [ll], the poison is oxidized and eliminated by potential pulses which maintain the electrode surface structure unaltered during the recording of the voltammetric current which is sampled after every pulse. The
I
I
#
I
I
0.5
I
I
I
I
E/V
(rhe)
Fig. 1. Stationary voltammogram of HCOOH oxidation on Pt(lll) in 0.5 M H,SO, voltammogram (-). Pulse voltammetry conditions: sampling window time: 0.38 s; pulse duration: 0.7 s. Potential of the pulse: 0.9 V; time interval between the end of the pulse and opening of the sampling window: 0.03 s. Sweep rate: 20 mV s-‘.
pulsed ( -), poison oxidation poison oxidation
232
corresponding voltammogram presents only a low increase of the current density (Fig. 1 dashed line) as compared with the conventional voltammetric sweep (solid line). In the latter case poison accumulates slowly on the surface for both directions of sweeping except above 0.72 V where the pulse voltammogram merges with the usual one, independent of the sweep direction for the latter. The current pulse profiles suggest that the deactivation process of the Pt(ll1) surface is slow. A comparison with the Pt(lOO) surface for the same experimental conditions [ll], where a fast decrease of the current pulse is observed, reinforces this conclusion. This result agrees with previous observations in which the Pt(ll1) surface presents the maximum ‘degree of resistance against poisoning’ [2]. On the other hand, this experiment also indicates the low intrinsic activity of the (111) orientation, compared to Pt(lOO), for formic acid oxidation via the active intermediate. Thus, the previous assumption [2] is confirmed that the Pt(ll1) orientation which presents the most densely packed atomic arrangement of all platinum faces is characterized by a low activity for the two reaction paths in formic acid oxidation. Behaviour of As precovered Pt(li1) electrodes for formic acid oxidation As has been shown [lo], arsenic can be adsorbed irreversibly on Pt(ll1). The redox process undergone by the adatom gives rise to reversible voltammetric oxidation-reduction peaks. The adsorbate valence change with formation of an arsenic oxygenated adspecies occurs in a narrow potential range. The potential range where the voltammogram of the Pt(lll)-As electrode is usually recorded, 0.06-0.75 V, is a potential range which corresponds to the stability domain of the adatom on the Pt(ll1) oriented electrode. The maximum arsenic coverage relative to the platinum surface atoms of the substrate is l/3 as shown in ref. 15. Because each arsenic atom blocks three hydrogen sites such a coverage gives a complete blocking of hydrogen adsorption. In other words, this means that if coverage is defined relative to the blocked hydrogen sites the alternative maximum value could be 1. The latter definition was that used in ref. 10. In order to distinguish clearly between the two possible definitions of the coverage (0) by an adatom (here As), O,, or O,,(H) will be used, if the coverage refers to platinum surface atoms or to adsorbed hydrogen, respectively. In the region of low arsenic coverages, the voltammogram (Fig. 2a) shows clearly the reversible redox surface process of the adatom with a peak potential at 0.58 V and hydrogen adsorption on the surface platinum sites free of adatoms. Typical features of hydrogen adsorption-desorption on the partially blocked Pt(ll1) sites are clearly visible, mainly regarding the distribution between the unusual hydrogen adsorption state, which is shifted positively with increasing amounts of adatoms [13], and the weakly bonded hydrogen state. In the same way, at sufficiently low arsenic coverages, the rest of the sharp peaks corresponding to the surface phase transition of adsorbed anions can be observed, and the related adsorption hydrogen isotherm change [16] which is characteristic of this platinum orientation in sulphuric acid solutions. Figure 2b shows the catalytic effect of adsorbed arsenic on the formic acid
233
I /pA
cm**
(4
I
IO -
i/mA
0.1
M
cm-*
E/V
(rhe)
Fig. 2. (a) (- - -) Voltammogram of the Pt(ll1) surface. () Voltammogram of the surface modified after the irreversible adsorption of arsenic /I,, = 0.11. Test solution: 0.5 M H,SO,; o = 50 mV s-‘. (b) () first voltammogram of HCOOH oxidation obtained with the same modified electrode as in (a), in a 0.25 M HCOOH+0.5 M H,SO, solution. (---) Stationary voltammogram of HCOOH oxidation on Pt(ll1) between the same potential limits.
oxidation reaction with the modified electrode giving the voltammogram (solid line) reported in Fig. 2a. In this figure a comparison of the voltammogram of the modified electrode with that yielded by pure Pt(ll1) (Fig. 2b dashed line) shows a current increase in the potential range below 0.6 V. It is interesting to observe that there is an increase in the activity in the potential range where the adatoms are in their reduced form and that the current suddenly falls when adatom oxidation takes place. The sequence is reversed with the direction of sweeping. The sudden reversible change of activity between 0.54 and 0.6 V is closely correlated with the reversible change of the valency state of the adatom observed in Fig. 2a. In the potential region where heteroatoms are in the oxidized state, the current reaches values always lower than those obtained on pure Pt(ll1) and independent of the direction of sweeping. It should be pointed out that the negativeand positive-going sweeps in the whole range of potential practically coincide. The
234
I /pA
(4
cmd2
\
1 I/mA
cm-*
M
5t
3-
l_
O-
, 0.1
0.5
E/V
(rhe)
Fig. 3. (a) Voltammogram of a Pt(lll)-As modified electrode at high coverage O,, = 0.31 in the test solution 0.5 M H,SO,; v = 50 mV s-‘. (b) First voltammetric cycle obtained with the same modified electrode as in (a) in 0.25 M HCOOH+0.5 M H,SO,; v = 50 mV s-l.
voltammogram is very stable with cycling. Holding the voltammetric sweep at the potential of maximum current maintains a nearly constant oxidation rate for formic acid. Moreover, it is possible to observe, for some coverages, that the current even increases slightly with the holding time. The redox peaks of the irreversibly adsorbed arsenic split when arsenic coverage approaches a monolayer (Fig. 3a) on Pt(ll1). The hydrogen charge decreases
235
proportionally, as studied in ref. 15, confined in the zone below 0.2 V, while the unusually bound hydrogen state vanishes. The formic acid oxidation reaction on these highly covered surfaces yields greater oxidation peak currents than described for lower arsenic coverages. However, the beginning of formic acid oxidation is shifted to more positive potential values. As a consequence of this the ascendent branch becomes concave and the oxidation peak narrows progressively with coverage (Fig. 3b). The potential of the current maximum remains constant and coincides with the valence state change of the adatom. The oxidation of formic acid at the lower potential range is inhibited slightly by increasing amounts of adsorbed arsenic. As in the case of low arsenic coverage, this voltammogram shows practically no hysteresis and the small effect observed between 0.54 and 0.60 V is easily explained in terms of the slight irreversibility in the surface redox reaction of the adatom. It should be pointed out also that the voltammetric current is quite stable with cycling. During arrests of the sweep in the ascendent and descendent branches of the voltammogram the current was constant. Slight current decreases were observed with arrests at the potential of the voltammogram apex while at lower coverages a slight increase has already been mentioned. Inhibition of poison intermediate reaction path Attempts at isolating the surface poison formed by dissociative adsorption [12,13] have failed even at low arsenic coverages. Figure 4 shows the nearly unchanged voltammetric profile after trying as usual to form the poison on an arsenic electrode of low coverage using a 0.25 M HCOOH + 0.5M H,SO, solution. No significant variation of the amount of hydrogen adsorbed on the As modified electrode could be detected either after exposure to formic acid or after a cycle in a region where the poison could be desorbed from the surface. On the other hand, no oxidation current in the potential range of the poison oxidation was detected. This result is surprising in view of the large number of hydrogen free sites or unblocked surface platinum atoms. The voltammetric profile suggests the existence of large Pt(ll1) domains on the partially arsenic covered electrode more than 12 platinum atom diameters wide in their lower dimension [17]. The impossibility of detecting the poisoning species on As precovered Pt(ll1) electrodes for O,, > 0.09 (minimum coverage studied in this work) shows that there is a large range of coverage for which pathway 2 does not affect the reaction pathway 1. This observation is similar to that obtained with irreversibly adsorbed bismuth on Pt(ll1) [8], where it was not possible to detect the presence of the poisoning intermediate from dissociative adsorption experiments performed under the same conditions as in the case of arsenic. This absence of poison on the surface could be inferred from the coincidence between the negative and positive sweeps in the voltammograms of the electrocatalytic oxidation of the formic acid at different arsenic coverages (Figs. 2b and 3b). The loss of the efficiency of pathway 2 for the blocking of reaction pathway 1 in the presence of arsenic on the Pt(ll1) surface deserves a special examination. It would be interesting to know how the adsorbed adatoms affect the inhibition of
236
a
Fig. 4. Negative voltammetric test for poison accumulation on the Pt(ll1) electrode at low arsenic coverage O,, = 0.09. (a) (- - -) Before poison formation; (b) (-) after exposure to the HCOOH solution, with different upper potential limits. Dissociative adsorption experiment attempted from 0.1 M formic acid in 0.5 M H,SO,; u = 50 mV SK’. Test solution 0.5 M H,SO,, inmersion potential 0.2 V.
pathway 2. The first possibility would be that poison cannot be formed by formic acid dissociation on the As-Pt(ll1) modified surface. A second possibility would be that poison cannot be accumulated on the modified surface after its formation assuming that dissociative adsorption of formic acid yields CO,, directly. In an attempt to clarify this characteristic behaviour of the As-Pt(ll1) modified electrode some experiments have been carried out to check whether CO could be adsorbed from a 0.5 M H,SO, solution using the same routine experiment as that described with formic acid solutions. The positive test for the detection of the adsorbed CO on a partially arsenic covered Pt(ll1) electrode is presented in Fig. 5. In this experiment almost complete blocking of the hydrogen adsorption sites is achieved after the exposure of the electrode to the CO solution (curve a). In the subsequent negative-going sweep the adsorption of hydrogen ability of the modified surface is restored (curve b). It should be pointed out that the Pt(lll)-As electrode used was initially covered more than the one used in the dissociative adsorption experiment (Fig. 4). Comparison of curves a and b in Fig. 5 shows that in cycle b the arsenic surface redox
237
Fig. 5. Positive voltammetric test for adsorbed CO detection from a CO saturated 0.5 M H,SO, on Pt(lll)-As electrode (As = 0.11). (a) After CO adsorption; (b) after CO oxidation-desorption.
solution
reaction is restored as before CO adsorption. This means that (i) hydrogen adsorption at full coverage of the arsenic free sites has played a role in the restoration process (ii) the oxidation of adsorbed arsenic occurs at lower potentials in the absence of adsorbed CO than when the surface is saturated indicating that adsorbed CO protects AS,~ against oxidation. On the other hand, after CO oxidation the amount of arsenic on the surface is slightly decreased because the upper potential limit required for complete CO oxidation is just outside of the potential range of stability of Asad. As a consequence, from the CO adsorption experiment it seems evident that the adatom acts on the Pt(ll1) surface through the inhibition of the poison formation reaction path because after its formation the poison can be accumulated on the surface. Moreover, the result of the experiment of CO adsorption has been confirmed even in the presence of 1 M formic acid in the CO containing solution. Apex voltammetric current densities and current densities obtained at 0.35, 0.45 and 0.5 V have been plotted versus the arsenic coverage (Fig. 6). It should be noted that the maximum current measured at 0.54 V increases progressively, showing a linear dependence on the surface arsenic concentration. This result points out that the surface completely blocked for hydrogen adsorption by arsenic can oxidize formic acid at the highest rate, nearly 1.5 times that corresponding to the intrinsic activity value of Pt(lll), obtained by pulse voltammetry. Nevertheless, currents measured at lower potentials show different optimum surface adspecies concentrations. In this way, current densities increase first with 8,, until a maximum current density is attained. From these maximum values a higher heteroatom coverage leads to a progressive inhibition of the ascendent branch of the voltammetric peak of formic acid oxidation reaction. The position of this maximum in the I-O,, curve is potential dependent: at 0.35 V the optimum coverage is obtained at @,, = 0.1-0.16 while at 0.5 V it is obtained around 0.27. This shows a progressive inhibition with
238
Fig. 6. (0) Plot of maximum current densities vs. arsenic coverage measured in the first voltammetric sweep of formic acid oxidation at 0.54 V. Current density vs. arsenic coverage at different potentials: (v) 0.35, (0) 0.45 and (A) 0.50 V. (- - -) Position of intrinsic activity of Pt(ll1) at @,, = 0.
increasing
arsenic
coverages
of
the
formic
acid
oxidation
current
in the low
potential range. DISCUSSION
The effect of adsorbed arsenic on the poison formation even at a low coverage similar to that of bismuth, suggests that these adatoms have a large electronic effect on the electrocatalytic properties of Pt(ll1). In the case of bismuth a large decrease of the work function with bismuth coverage has been reported [18]. A similar effect may be expected with As adatoms in view of the large inhibiting effect on the dissociative adsorption reaction. This means that surface platinum atoms of Pt(ll1) free of arsenic may have their catalytic activity modified considerably by the presence of small amounts of arsenic on the surface, as regards poison formation. The results reported in Fig. 6 in the absence of poisoning intermediate are thus relative to the direct oxidation pathway. They can be explained through the existence of two types of independent reactions for the formic acid oxidation observed predominantly at the beginning of oxidation and near the maximum of the voltammetric current, respectively. The first one would take place in the low potential zone and have its optimum arsenic coverage at low arsenic coverages. For this reason it may be assumed that the reaction occurs on the surface domains free of arsenic, which represent the major part of the surface. The second one would proceed at higher potentials and its rate increases with increasing arsenic coverages. This indicates that the reaction may
239
takes place on the surface domains where arsenic is adsorbed. This is in agreement with the fact that at maximum arsenic coverages the current density is also at a maximum. All the values reported in Fig. 6 are relative to surface conditions where the adatoms are in their reduced state (zero valency state). The combination of both contributions along the ascendent branch of the voltammogram may explain the variation of the optimum coverage value in the lower potential range as reported above and the linear activity increase of the electrode with arsenic coverage in the peak potential region. The maximum of activity for formic acid oxidation is observed when arsenic is in its reduced form below 0.54 V; the activity at all arsenic coverages falls when arsenic is present on the surface in a combination with oxygen, and becomes lower than that obtained on pure Pt(ll1) electrodes. The chemical change due to surface arsenic oxidation decreases the catalytic properties of the overall surface sample considerably and probably induces an inhibiting effect on those platinum surface atoms that are combined with the oxygenated arsenic surface compound.
CONCLUSIONS
Experiments carried out on CO adsorption from a CO saturated solution show that the arsenic present on Pt(ll1) surfaces inhibits the poison formation reaction path in formic acid oxidation and that it does not block the adsorption of the reaction product. This phenomenon lies at the origin of the negative results obtained for isolation of the poison formed from formic acid dissociative adsorption experiments on such modified surfaces. Nevertheless, when CO has been adsorbed from the CO saturated solution the oxidation of CO gives rise to a complex voltammetric profile in the potential range where As is combined with oxygen. CO,, appears to protect adsorbed As against oxidation. It cannot be excluded that this compound participates in the incipient oxidation of adsorbed CO by providing oxygen atoms necessary for CO, formation. This result is similar to that observed for bismuth irreversible adsorption on the Pt(ll1) surface [8] and it suggests a considerable change in the electrocatalytic properties of the As modified surface which is significantly more selective and active for the reaction pathway via the active intermediate. The maximum activity at O,, = l/3 was found to be 1.5 times higher than the intrinsic activity measured at O,, = 0. In both cases the potential is the same (0.54 V). The analysis of the formic acid oxidation current at different potentials and coverages suggests the presence of two types of reaction taking place: on the one hand at the arsenic free platinum surface domains and on the other hand at those covered by the adatoms. Both reactions combined together give the oxidation current of the overall process. The former would be predominant at the beginning of formic acid oxidation while the latter would be predominant when the oxidation current peak reaches its maximum value. A marked decrease in the activity is
240
observed when AS,,, is oxidized on the surface, forming an oxygenated compound.
surface
ACKNOWLEDGEMENTS
The financial support of the DGICYT through project No. PB87-0795 is gratefully acknowledged. One of us, J.C., is grateful for a stay at the University of Alicante in the framework of the “Mercure” programme for scientific exchange. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
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