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Oxidation of formic acid on Pt(111) electrodes modified by irreversibly adsorbed tellurium
JOURNAL OF
ELSEVIER
Journal of Electroanalytical Chemistry 394 (1995) 161-167
Oxidation of formic acid on Pt(111) electrodes modified by irreversibly adsorbed tellurium E. Herrero, M.J. Llorca, J.M. Feliu, A. Aldaz Departament de Qulmica Ffsica, Universitat d'Alacant, Ap. Correus 99, E-03080 Alacant, Spain Received 8 November 1994; in revised form 14 February 1995
Abstract The electrocatalytic behaviour of tellurium-modified Pt(lll) electrodes for the oxidation of formic acid was investigated. For the direct oxidation of formic acid, an enhancement of catalysis was observed, yielding a maximum formic acid oxidation current for a tellurium coverage of 0.24. In poison formation experiments, the tellurium adsorbed on the surface inhibited poison formation and accumulation by an electronic effect. A comparison was made with other s2p " adatoms. It was found that the elements which have a lower work function than platinum inhibit poison formation through a long-range electronic effect when they are adsorbed on the Pt(ll 1) electrode. Keywords: Formic acid oxidation; P t ( l l l ) electrode; tellurium- modified electrode
1. Introduction The oxidation of formic acid on platinum electrodes is a model reaction for the understanding of structural effects in electrocatalysis. The oxidation of this small molecule on a platinum single crystal surface displays an important dependence on the surface structure of the single crystal electrode since both direct oxidation and surface poisoning are structure-sensitive processes. On Pt(100) electrodes, poisoning of the electrode is very fast and the direct oxidation of the molecule cannot take place until the poison is removed from the surface at high potential values [1]. In the absence of poison on the surface, the intrinsic activity of the Pt(100) electrode for the formic acid oxidation reaction reaches high values (27 m A c m -2 at 0.4 V for 0.1 M HCOOH in 0.5 M H2SO 4) [2]. Conversely, the oxidation of formic acid on P t ( l l l ) electrodes shows negligible poisoning under voltammetric conditions [1]. However, the direct oxidation rate is also small [3] (3 mA cm -2 in pulsed voltammetric experiments). It is well known that the presence of foreign adatoms on platinum surfaces can be used to enhance the direct oxidation of formic acid. In order to understand the origin of this enhancement, experiments were performed using well-defined substrates, since the same adatom can play different roles according to the single crystal plane on 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSD1 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 3 9 6 3 - 5
which the adatom is adsorbed [4,5]. Different adatoms deposited on the same single crystal electrodes can modify the electrode properties in different ways. Since formic acid oxidation is a structure- sensitive reaction, the effect of these adatoms on the surface may also be different. Hence s2p" adatoms show a simple third body effect when they are adsorbed on Pt(100) electrodes [5]. These adatoms block the Pt(100) surface and inhibit poison formation and thus the remaining free sites can display high activity. This implies that the poison formation path involves more platinum sites than those required for the direct oxidation of formic acid. The formation of ensemble sites with appropriate dimensions (probably involving a single isolated Pt(100) site) is the key to the explanation of the electrocatalytic effect observed. On P t ( l l l ) electrodes, the situation is different. If arsenic or bismuth is adsorbed on the P t ( l l l ) surface, there is an important inhibition of the poison formation reaction at very low adatom coverages [4]. Poison formation is negligible at adatom coverages for which 85% of the platinum sites are still uncovered. Moreover, these modified surfaces can oxidize formic acid at higher rates than the intrinsic activity of the P t ( l l l ) surface. These observations suggest that an electronic effect is responsible for the observed electrocatalysis of formic acid oxidation. However, selenium adatoms exhibit no electronic effect,
162
E. Herrero et al. /Journal of Electroanalytical Chemistry 394 (1995) 161-167
since the maximum formic acid oxidation current is similar to the intrinsic activity and poison formation is inhibited through a third body effect. This paper deals with formic acid oxidation on P t ( l l l ) substrates modified by irreversibly adsorbed tellurium. The investigation of this modified electrode is used to obtain a general trend in the behaviour of s2p n adatoms on this substrate, and enables a general model to be proposed of the effect of these adatoms on formic acid oxidation on Pt(111) electrodes.
2. E x p e r i m e n t a l
The electrodes, apparatus and cells have been described elsewhere [6]. Platinum single crystal electrodes were prepared from platinum beads according to the technique developed by Clavilier et al. [7]. The chemicals used in this work were: suprapur sulphuric acid (Merck), pro analysi formic acid (Merck), TeO 2 (Merck) and ultrapure Millipore MilliQ water. All potentials were measured against a reversible hydrogen electrode (RHE). The coverage was defined as the number of adsorbed species per surface platinum atom. The irreversible adsorption of tellurium was carried out on quenched and annealed P t ( l l l ) electrodes as described in Ref. [8]. The modified electrodes were characterized in 0.5 M H 2 S O 4 solutions by controlling the amount of adsorbed adatoms and the remaining free platinum sites. Irreversibly adsorbed tellurium gives a surface redox peak at 0.83 V, and the charge transferred in the oxidation process of the adatom was used to calculate the tellurium coverage. Assuming that four electrons are exchanged per tellurium adatom in its surface redox process [8], the tellurium coverage, OTe, can be evaluated using the following equation Ore = ¼QTe/241 /zC cm -2
the experimental protocol have been described elsewhere [9,10].
3. R e s u l t s
3.1. Electrocatalytic results for low tellurium coverages on P t ( l l l ) electrodes (Ore < 0.10) Small amounts of tellurium adsorbed on a P t ( l l l ) electrode have an influence on formic acid oxidation (Fig. 1). For unmodified P t ( l l l ) electrodes, the currents in the positive-going scan are always lower than in the negativegoing scan [11], since poison is formed on the electrode surface and blocks part of the surface for the direct oxidation of formic acid. When small amounts of tellurium are adsorbed on the surface, currents in the positive-going sweep are always higher than in the negative-going scan. Thus there is no deactivation of the surface because of poison formation, which implies that very small amounts of poison are formed in every cycle. Unlike Se-Pt(lll), where no increase in the maximum oxidation current is found for low selenium coverages with respect to that of the unmodified P t ( l l l ) electrode [9], an increase in the current in the presence of tellurium was observed for the present system. However, the onset of oxidation of formic acid is displaced towards higher potentials.
50
<
0
0.9
,,..-n
E/V (RHE)
(1)
where Qve is the charge involved in the surface redox process of adsorbed tellurium, four is the number of electrons exchanged by each tellurium adatom and 241 /xC cm -2 is the charge corresponding to a process in which every platinum atom on a P t ( l l l ) electrode adsorbs a species that exchanges an electron on adsorption. At a tellurium coverage of 0.25, all the platinum adsorption states are blocked by the adatoms. For Oxe > 0.25, the calculation of the tellurium coverage is more difficult, since the redox peak of the adsorbed tellurium becomes ill defined and the charge measurements are inaccurate. In order to obtain an approximate coverage, the procedure described in Ref. [8] was used. When tellurium species are present in solution, the coverage is increased linearly with the voltammetric cycles. This linear increase is used to establish a relationship between the redox charge of the tellurium peak and the tellurium coverage. The details of
V
(A)
0
L ?
"~
!
0.5 (B)
0.9 E/V ( R H E )
Fig. 1. (A) Voltammogram of a P t ( l l l ) electrode with @'r~ = 0.05 in 0.5 M H2SO 4. (B) Voltammogram of the previous electrode in 0.5 M H2SO 4 +0.1 M HCOOH. Sweep rate is 50 mV s -1 in all figures.
E. Herrero et al. /Journal of Electroanalytical Chemistry 394 (1995) 161-167
tion does not require oxygen adsorption on the surface as observed with other adatom-modified electrodes and the substrate itself [3,9-15].
0.5
• v'-
|
t
|
0.5
//0.9
E/V (RHE) (A
4
.,--n
0 (B)
163
0.5 0.9 E/V (RHE)
Fig. 2. (A) Voltammogramof a Pt(lll) electrode with ~gTe= 0.22 in 0.5 M H2SO 4. (B) Voltammogramof the same electrode in 0.5 M H2SO 4 q0.1 M HCOOH.
3.2. Electrocatalytic results for intermediate tellurium coverages on Pt(111) electrodes (0.10 < (gre < 0.25) An increase in tellurium on the P t ( l l l ) electrode surface leads to a continuous increase in the oxidation current found in the positive-going scan (Fig. 2). Together with a current increase, there is a small shift of the potential of the maximum current towards positive potentials. At tellurium coverages of 0.22, a shoulder starts to appear at 0.75 V in the descending branch in the voltammetric profile after the maximum in the positive-going scan (Fig. 3). This shoulder grows as the tellurium coverage increases (Fig. 4) and, when the maximum oxidation current is achieved at a tellurium coverage of 0.24, the peak potential corresponds to that of the shoulder. The maximum current is 6.1 /xA cm 2, twofold higher than the intrinsic oxidation current found for unmodified P t ( l l l ) electrodes [3]. It is also interesting to note that, for these coverages, the redox peak of adsorbed tellurium is clearly visible in the formic acid oxidation voltammogram at the same potential as observed in the test electrolyte. The presence of formic acid in solution does not alter significantly the redox behaviour of the adatom. However, the oxidation state of the adatom has an influence on the oxidation of formic acid. At potentials at which the tellurium adatom is in its oxidized form, no significant oxidation currents are found. Therefore this oxidized form seems to inhibit the oxidation of formic acid. As expected, formic acid oxida-
3.3. Electrocatalytic results for high tellurium coverages on Pt(111) electrodes (Ore > 0.25) At tellurium coverages greater than 0.25, all platinum adsorption states are blocked by adsorbed tellurium. However, formic acid oxidation still occurs at tellurium coverages lower than 0.40. The oxidation current is relatively high ( 4 - 5 /xC cm -2) but, with the onset of formic acid oxidation, is displaced towards positive potentials compared with lower tellurium coverages (Fig. 5). Tellurium adatoms are stable on the electrode surface at potentials lower than 0.9 V. However, if the upper potential limit is increased to 1.0 V, desorption occurs [8]. In Fig. 6, the upper potential limit for the oxidation of formic acid was set at 1.0 V and, for simplicity, only positive-going scans are reported. As can be seen, tellurium desorption leads to an increase in the oxidation current at low potentials in successive cycles as the tellurium adatoms are desorbed from the surface. The maximum currents for formic acid oxidation obtained in the positive-going scan for different tellurium
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!
I
--
O.5
E/V (RHE)
-
19
(A)
4
2
o (B)
0.5
o~9 E/V (RHE)
Fig. 3. (A) Voltammogramof a Pt(111) electrode with @ve = 0.23 in 0.5 M H2SO 4. (B) Voltammogramof the same electrode in 0.5 M H2SO 4 + 0.1 M HCOOH.
E. Herrero et al. /Journal of Electroanalytical Chemistry 394 (1995) 161-167
164
0.5 0.5
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3 ~.9
cJ
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I
I
i
-
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E/V (RHE) E/V (RHE) (A)
(A
6 4 4
42 0
2
0.5
(B)
0.9 E/V (RHE)
Fig. 4. (A) Voltammogramof a Pt(lll) electrode with OTe = 0.24 in 0.5 M H2SO4. (B) Voltammogramof the same electrode in 0.5 M H 2 S O 4 d0.1 M HCOOH.
0
!
0.5 0.9 E/V (RHE)
(B)
coverages are reported in Fig. 7. The current values for high coverages are represented as open circles.
3.4. Poison formation experiments It has been shown that poison formation for low tellurium coverages is rather small during the voltammetric scan. This effect may be a consequence of either a decrease in the poison formation reaction rate or a complete inhibition of poison formation. In the first case, poison would still accumulate on the surface to a significant extent, but in a voltammetric scan at 50 mV s -1 the amount accumulated would be negligible. However, the general behaviour observed in the experiments resembles that found for bismuth- and arsenic-modified P t ( l l l ) electrodes [3,13], where an electronic effect was found [4]. In order to prove this hypothesis, long-term poison formation experiments were carried out at open circuit. Fig. 8 shows the results obtained for a tellurium coverage of 0.015. For the unmodified surface, the maximum amount of poison that can be accumulated corresponds to an oxidation charge (Qpi) of 1 9 2 / z C cm -2 [4]. In the presence of this very low amount of adsorbed tellurium, Qpi = 18 /zC cm -2, which is just one-tenth of the value obtained for the unmodified surface. Such an
Fig. 5. (A) Voltammogramof a Pt(111) electrode with OT~ = 0.30 in 0.5 M H 2 S O 4. (B) Voltammogramof the same electrode in 0.5 M H 2 S O 4 40.1 M HCOOH.
important inhibition cannot be explained through a simple third body mechanism, since the amount of adatoms on the surface is too low. It appears that the only possible mechanism is an electronic effect of the adsorbed adatom on the neighbouring P t ( l l 1) domains.
2
4 A.
0
0.5 0.9 E/V (RHE)
Fig. 6. Voltammogramof a Pt(111) electrode with Oxe ----0.4 in 0.5 M H 2 S O 4 4- 0.1 M HCOOH with partial desorptionof telluriumat the upper potential limit. Only positive-goingscans are shown. Arrows indicatethe evolution of the voltammetricprofile.
165
E. Herrero et al. /Journal of Electroanalytical Chemistry 394 (1995) 161-167
200
150 0
0
,?,
4
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~,~ 100 gl.
3
..,..... °~
50
0 0.0
I
i
i
I
0.1
0.2
0.3
0.4
i
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Or,
Fig. 7. Maximum current densities for the oxidation of formic acid obtained in the first positive-going scan vs. tellurium coverage in 0.5 M
0 0.00
0.01
0.02
0.03
0.04
0.05
Ox, Fig. 9. Maximum stripping charge of COad~ originating from poison formation reaction from formic acid.
U2SO 4.
Plotting the oxidation charge of the poison intermediate vs. the tellurium coverage (Fig. 9) leads to a very similar plot as obtained for arsenic and bismuth [4]. In the latter case, it has been estimated that every bismuth adatom impedes poison formation in all platinum atoms at a distance lower than six atoms from an adatom. The similarities between the two systems indicate that the tellurium adatoms on the surface reach a comparable number of platinum atoms. The general behaviour of tellurium against poison formation is comparable with that observed in the
50
d
<
-1
Fig. 8. Poison stripping experiment on Te + Pt(111) electrodes for formic acid in 0.5 M H2SO 4 with ~9.r¢= 0.02: (a) voltammogram of the electrode before the dissociative adsorption experiment; (b) adsorption profile of the electrode blocked by the poison; (c) poison stripping; (d) recovery of the adsorptive properties of the electrode after poison stripping (broken line).
case of bismuth and arsenic [4] and different from that reported in the case of selenium [9].
4. Discussion The four adatoms studied (Bi, As, Se and Te), when adsorbed on P t ( l l l ) , yield two different inhibition mechanisms for the poison formation reaction from formic acid, i.e. an important long-range electronic effect has been found for bismuth, arsenic [4] and tellurium at very low adatom coverages, whereas a short-range third body effect has been observed for selenium-modified electrodes, where the amount of poison decreases linearly with selenium coverage [9]. In the cases in which an electronic effect was found, it extended over a long distance and affected the platinum atoms within 6 - 7 rows around the adatom, which is probably randomly adsorbed on the P t ( l l l ) substrate at low coverages. Model calculations of the influence of an adatom on the Fermi level density of states indicates that the perturbation of an adatom extends further than the next-nearest neighbour to the adatom [16], but there is no indication that the influence could reach such a distance. In our case, the great extent of the inhibition effect may be the joint result of the modification of the electronic properties of the surface and its influence on the double layer properties. The modification of the double layer properties induced by the adsorbed adatom includes a change in the water adsorption modes and anion adsorption. The change in anion adsorption probably does not affect poison formation since it takes place in the low potential range where anions are desorbed [17]. This modification may extend further than the pure electronic effect if ice-like adsorbed
166
E. Herrero et aL /Journal of Electroanalytical Chemistry 394 (1995) 161-167
water layers are considered [18]. Thus a single adatom may disrupt the ordered water adlayer, and extend its effect over a long distance. In order to understand better the difference in the behaviour of the adatoms studied, a comparison of the electronic properties of the adatoms and platinum was performed. As a first approach, we compared the electronegativities of the adatoms and platinum. Tellurium has the same electronegativity as platinum (2.1), selenium has a higher electronegativity (2.4), and arsenic and bismuth have lower electronegativities (2 and 1.8 respectively) [19]. Taking into account the observed inhibition of the poison formation reaction, adatoms with a lower electronegativity than platinum give an electronic effect on poison formation and adatoms with a higher electronegativity than platinum give a third body effect. For tellurium, the behaviour is not clear. Electronegativities are assigned for non-condensed environments. Therefore it may be possible that the electronegativities do not reflect the electronic properties when the atoms are in the solid state. A better approach would be to consider the work function values. Selenium has a higher work function than platinum (5.9 vs. 5.6 eV) and the three other adatoms have a lower work function (4.22 eV for Bi, 4.95 eV for Te and 4.9-5.4 eV for As) [20]. These data agree well with the inhibition effects found for the different adatoms. Thus adatoms displaying a lower work function than platinum modify the properties of the surface in such a way that an important inhibition through an electronic effect is found. For the elements that have a higher work function than platinum, the modifications of the surface properties do not alter the behaviour of the surface with respect to CO formation from formic acid and only a third body effect is found. This qualitative approach does not take into account the work function of the P t ( l l l ) substrate (6.1 eV [21]), and is only based on the mean values obtained for polycrystalline materials. However, it is implicit in the approach that the work function of the adatoms parallels the behaviour observed for the substrate, i.e. a layer of the adatom on the P t ( l l l ) electrode will also have a higher work function than that obtained for the polycrystalline material. It is clear that more experiments with other adatoms under the same controlled conditions are needed to corroborate the relationship proposed between the inhibition mechanism for poison formation and the work function of the adatom. The difference from the behaviour obtained on the Pt(100) substrate, where the long-range order effect is not observed, is probably related to the different bonding of the adatom to the surface [22] and the ability of the surface to form a water adlayer. It is known that an ordered water adlayer is not formed on f.c.c. (100) surfaces in vacuum [18]. The absence of the ordered adlayer is probably linked to the faster poisoning of this latter electrode compared with the Pt(ll 1) electrode. The adatoms cannot disturb the water adlayer, since it is probably not formed on this
plane, and the effect of the adatoms is restricted only to the nearest neighbours (third body effect). It would also be interesting to find a correlation with the catalysis of direct formic acid oxidation. This is a more complicated issue since formic acid oxidation in the presence of adatoms does not always occur in the same potential range at different adatom coverages. However, a trend can be observed in the maximum currents obtained for formic acid oxidation: Se [9] < Te < As [3] < Bi [13]. Selenium has the highest work function, and the maximum current found is approximately the same as the intrinsic activity of platinum. At the other extreme, bismuth has the lowest work function and the highest catalytic activity. This provides a qualitative picture of the effects of the adatoms on the direct oxidation of formic acid, i.e. a higher catalytic activity is obtained for elements having a lower work function than platinum when adsorbed on P t ( l l l ) . Nevertheless, a quantitative approach needs a more detailed knowledge of how the adatom changes the energy levels of platinum and to what extent.
Acknowledgements This work was carried out within the framework of the DGICYT project PB 93-0944. J.M.F. acknowledges fruitful discussions with Dr. T. Iwasita. E.H. and M.J.L. acknowledge the award of grants by the Conselleria de Educaci6 i Ci~ncia of the Generalitat Valenciana and Fundaci6n C.A.M. respectively.
References [1] J. Clavilier, R. Parsons, R. Durand, C. Lamy and J.M. Leger, J. Electroanal. Chem., 124 (1981) 321. [2] J. Clavilier, J. Electroanal. Chem., 236 (1987) 87. [3] A. Fernandez-Vega, J.M. Feliu, A. Aldaz and J. Clavilier, J. Electroanal. Chem., 305 (1991) 229. [4] E. Herrero, A. Fernandez-Vega, J.M. Feliu and A. Aldaz, J. Electroanal. Chem., 350 (1993) 73. [5] E. Herrero, J.M. Feliu and A. Aldaz, J. Electroanal. Chem., 368 (1994) 101. [6] J. Clavilier, J.M. Feliu and A. Aldaz, J. Electroanal. Chem., 243 (1988) 419. [7] J. Clavilier, D. Armand, S.G. Sun and M. Petit, J. Electroanal. Chem., 205 (1986) 267. [8] J.M. Feliu, M.J. Llorca, R. G6mez and A. Aldaz, Surf. Sci., 297 (1993) 209. [9] M.J. Llorca, E. Herrero, J.M. Feliu and A. Aldaz, J. Electroanal. Chem., 373 (1994) 217. [10] E. Herrero, M.J. Llorca, J.M. Feliu and A. Aldaz, J. Electroanal. Chem., 383 (1995) 145. [11] S.G. Sun, J. Clavilier and A. Bewick, J. Electroanal. Chem., 240 (1988) 147. [12] M. Shibata, O. Takahashi and M. Motoo, J. Electroanal. Chem., 249 (1988) 253, and references cited therein. [13] J. Clavilier, A. Fernandez-Vega, J.M. Feliu and A. Aldaz, J. Electroanal. Chem., 258 (1989) 89. [14] A. Fernfindez-Vega, J.M. Feliu, A. Aldaz and J. Clavilier, J. Electroanal. Chem., 258 (1989) 101.
E. Herrero et al. /Journal of Electroanalytical Chemistry 394 (1995) 161-167 [15] J. Clavilier, A. Fernandez-Vega, J.M. Feliu and A. Aldaz, J. Electroanal. Chem., 261 (1989) 113. [16] P.J. Feibelman and D.R. Ramman, Phys. Rev. Lett., 52 (1984) 61. [17] S.-C. Chang, Y. Ho and M.J. Weaver, Surf. Sci., 265 (1992) 81. [18] P.A. Thiel and T.E. Madey, Surf. Sci. Rep., 7 (1987) 211.
[19] [20] [21] [22]
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W. Gordy and W.J. Orville Thomas, J. Chem. Phys., 24 (1956) 439. H.B. Michaelson, J. Appl. Phys., 48 (1977) 4729. G.N. Derry and Z. Ji-Zhong, Phys. Rev. B, 39 (1989) 1940. N. Furuya and S. Koide, Surf. Sci., 220 (1989) 18.
ELSEVIER
Journal of Electroanalytical Chemistry 394 (1995) 161-167
Oxidation of formic acid on Pt(111) electrodes modified by irreversibly adsorbed tellurium E. Herrero, M.J. Llorca, J.M. Feliu, A. Aldaz Departament de Qulmica Ffsica, Universitat d'Alacant, Ap. Correus 99, E-03080 Alacant, Spain Received 8 November 1994; in revised form 14 February 1995
Abstract The electrocatalytic behaviour of tellurium-modified Pt(lll) electrodes for the oxidation of formic acid was investigated. For the direct oxidation of formic acid, an enhancement of catalysis was observed, yielding a maximum formic acid oxidation current for a tellurium coverage of 0.24. In poison formation experiments, the tellurium adsorbed on the surface inhibited poison formation and accumulation by an electronic effect. A comparison was made with other s2p " adatoms. It was found that the elements which have a lower work function than platinum inhibit poison formation through a long-range electronic effect when they are adsorbed on the Pt(ll 1) electrode. Keywords: Formic acid oxidation; P t ( l l l ) electrode; tellurium- modified electrode
1. Introduction The oxidation of formic acid on platinum electrodes is a model reaction for the understanding of structural effects in electrocatalysis. The oxidation of this small molecule on a platinum single crystal surface displays an important dependence on the surface structure of the single crystal electrode since both direct oxidation and surface poisoning are structure-sensitive processes. On Pt(100) electrodes, poisoning of the electrode is very fast and the direct oxidation of the molecule cannot take place until the poison is removed from the surface at high potential values [1]. In the absence of poison on the surface, the intrinsic activity of the Pt(100) electrode for the formic acid oxidation reaction reaches high values (27 m A c m -2 at 0.4 V for 0.1 M HCOOH in 0.5 M H2SO 4) [2]. Conversely, the oxidation of formic acid on P t ( l l l ) electrodes shows negligible poisoning under voltammetric conditions [1]. However, the direct oxidation rate is also small [3] (3 mA cm -2 in pulsed voltammetric experiments). It is well known that the presence of foreign adatoms on platinum surfaces can be used to enhance the direct oxidation of formic acid. In order to understand the origin of this enhancement, experiments were performed using well-defined substrates, since the same adatom can play different roles according to the single crystal plane on 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSD1 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 3 9 6 3 - 5
which the adatom is adsorbed [4,5]. Different adatoms deposited on the same single crystal electrodes can modify the electrode properties in different ways. Since formic acid oxidation is a structure- sensitive reaction, the effect of these adatoms on the surface may also be different. Hence s2p" adatoms show a simple third body effect when they are adsorbed on Pt(100) electrodes [5]. These adatoms block the Pt(100) surface and inhibit poison formation and thus the remaining free sites can display high activity. This implies that the poison formation path involves more platinum sites than those required for the direct oxidation of formic acid. The formation of ensemble sites with appropriate dimensions (probably involving a single isolated Pt(100) site) is the key to the explanation of the electrocatalytic effect observed. On P t ( l l l ) electrodes, the situation is different. If arsenic or bismuth is adsorbed on the P t ( l l l ) surface, there is an important inhibition of the poison formation reaction at very low adatom coverages [4]. Poison formation is negligible at adatom coverages for which 85% of the platinum sites are still uncovered. Moreover, these modified surfaces can oxidize formic acid at higher rates than the intrinsic activity of the P t ( l l l ) surface. These observations suggest that an electronic effect is responsible for the observed electrocatalysis of formic acid oxidation. However, selenium adatoms exhibit no electronic effect,
162
E. Herrero et al. /Journal of Electroanalytical Chemistry 394 (1995) 161-167
since the maximum formic acid oxidation current is similar to the intrinsic activity and poison formation is inhibited through a third body effect. This paper deals with formic acid oxidation on P t ( l l l ) substrates modified by irreversibly adsorbed tellurium. The investigation of this modified electrode is used to obtain a general trend in the behaviour of s2p n adatoms on this substrate, and enables a general model to be proposed of the effect of these adatoms on formic acid oxidation on Pt(111) electrodes.
2. E x p e r i m e n t a l
The electrodes, apparatus and cells have been described elsewhere [6]. Platinum single crystal electrodes were prepared from platinum beads according to the technique developed by Clavilier et al. [7]. The chemicals used in this work were: suprapur sulphuric acid (Merck), pro analysi formic acid (Merck), TeO 2 (Merck) and ultrapure Millipore MilliQ water. All potentials were measured against a reversible hydrogen electrode (RHE). The coverage was defined as the number of adsorbed species per surface platinum atom. The irreversible adsorption of tellurium was carried out on quenched and annealed P t ( l l l ) electrodes as described in Ref. [8]. The modified electrodes were characterized in 0.5 M H 2 S O 4 solutions by controlling the amount of adsorbed adatoms and the remaining free platinum sites. Irreversibly adsorbed tellurium gives a surface redox peak at 0.83 V, and the charge transferred in the oxidation process of the adatom was used to calculate the tellurium coverage. Assuming that four electrons are exchanged per tellurium adatom in its surface redox process [8], the tellurium coverage, OTe, can be evaluated using the following equation Ore = ¼QTe/241 /zC cm -2
the experimental protocol have been described elsewhere [9,10].
3. R e s u l t s
3.1. Electrocatalytic results for low tellurium coverages on P t ( l l l ) electrodes (Ore < 0.10) Small amounts of tellurium adsorbed on a P t ( l l l ) electrode have an influence on formic acid oxidation (Fig. 1). For unmodified P t ( l l l ) electrodes, the currents in the positive-going scan are always lower than in the negativegoing scan [11], since poison is formed on the electrode surface and blocks part of the surface for the direct oxidation of formic acid. When small amounts of tellurium are adsorbed on the surface, currents in the positive-going sweep are always higher than in the negative-going scan. Thus there is no deactivation of the surface because of poison formation, which implies that very small amounts of poison are formed in every cycle. Unlike Se-Pt(lll), where no increase in the maximum oxidation current is found for low selenium coverages with respect to that of the unmodified P t ( l l l ) electrode [9], an increase in the current in the presence of tellurium was observed for the present system. However, the onset of oxidation of formic acid is displaced towards higher potentials.
50
<
0
0.9
,,..-n
E/V (RHE)
(1)
where Qve is the charge involved in the surface redox process of adsorbed tellurium, four is the number of electrons exchanged by each tellurium adatom and 241 /xC cm -2 is the charge corresponding to a process in which every platinum atom on a P t ( l l l ) electrode adsorbs a species that exchanges an electron on adsorption. At a tellurium coverage of 0.25, all the platinum adsorption states are blocked by the adatoms. For Oxe > 0.25, the calculation of the tellurium coverage is more difficult, since the redox peak of the adsorbed tellurium becomes ill defined and the charge measurements are inaccurate. In order to obtain an approximate coverage, the procedure described in Ref. [8] was used. When tellurium species are present in solution, the coverage is increased linearly with the voltammetric cycles. This linear increase is used to establish a relationship between the redox charge of the tellurium peak and the tellurium coverage. The details of
V
(A)
0
L ?
"~
!
0.5 (B)
0.9 E/V ( R H E )
Fig. 1. (A) Voltammogram of a P t ( l l l ) electrode with @'r~ = 0.05 in 0.5 M H2SO 4. (B) Voltammogram of the previous electrode in 0.5 M H2SO 4 +0.1 M HCOOH. Sweep rate is 50 mV s -1 in all figures.
E. Herrero et al. /Journal of Electroanalytical Chemistry 394 (1995) 161-167
tion does not require oxygen adsorption on the surface as observed with other adatom-modified electrodes and the substrate itself [3,9-15].
0.5
• v'-
|
t
|
0.5
//0.9
E/V (RHE) (A
4
.,--n
0 (B)
163
0.5 0.9 E/V (RHE)
Fig. 2. (A) Voltammogramof a Pt(lll) electrode with ~gTe= 0.22 in 0.5 M H2SO 4. (B) Voltammogramof the same electrode in 0.5 M H2SO 4 q0.1 M HCOOH.
3.2. Electrocatalytic results for intermediate tellurium coverages on Pt(111) electrodes (0.10 < (gre < 0.25) An increase in tellurium on the P t ( l l l ) electrode surface leads to a continuous increase in the oxidation current found in the positive-going scan (Fig. 2). Together with a current increase, there is a small shift of the potential of the maximum current towards positive potentials. At tellurium coverages of 0.22, a shoulder starts to appear at 0.75 V in the descending branch in the voltammetric profile after the maximum in the positive-going scan (Fig. 3). This shoulder grows as the tellurium coverage increases (Fig. 4) and, when the maximum oxidation current is achieved at a tellurium coverage of 0.24, the peak potential corresponds to that of the shoulder. The maximum current is 6.1 /xA cm 2, twofold higher than the intrinsic oxidation current found for unmodified P t ( l l l ) electrodes [3]. It is also interesting to note that, for these coverages, the redox peak of adsorbed tellurium is clearly visible in the formic acid oxidation voltammogram at the same potential as observed in the test electrolyte. The presence of formic acid in solution does not alter significantly the redox behaviour of the adatom. However, the oxidation state of the adatom has an influence on the oxidation of formic acid. At potentials at which the tellurium adatom is in its oxidized form, no significant oxidation currents are found. Therefore this oxidized form seems to inhibit the oxidation of formic acid. As expected, formic acid oxida-
3.3. Electrocatalytic results for high tellurium coverages on Pt(111) electrodes (Ore > 0.25) At tellurium coverages greater than 0.25, all platinum adsorption states are blocked by adsorbed tellurium. However, formic acid oxidation still occurs at tellurium coverages lower than 0.40. The oxidation current is relatively high ( 4 - 5 /xC cm -2) but, with the onset of formic acid oxidation, is displaced towards positive potentials compared with lower tellurium coverages (Fig. 5). Tellurium adatoms are stable on the electrode surface at potentials lower than 0.9 V. However, if the upper potential limit is increased to 1.0 V, desorption occurs [8]. In Fig. 6, the upper potential limit for the oxidation of formic acid was set at 1.0 V and, for simplicity, only positive-going scans are reported. As can be seen, tellurium desorption leads to an increase in the oxidation current at low potentials in successive cycles as the tellurium adatoms are desorbed from the surface. The maximum currents for formic acid oxidation obtained in the positive-going scan for different tellurium
0.5
!
!
I
--
O.5
E/V (RHE)
-
19
(A)
4
2
o (B)
0.5
o~9 E/V (RHE)
Fig. 3. (A) Voltammogramof a Pt(111) electrode with @ve = 0.23 in 0.5 M H2SO 4. (B) Voltammogramof the same electrode in 0.5 M H2SO 4 + 0.1 M HCOOH.
E. Herrero et al. /Journal of Electroanalytical Chemistry 394 (1995) 161-167
164
0.5 0.5
S xl0
o
.)
~"
0.5
3 ~.9
cJ
<
I
I
i
-
0.5
E/V (RHE) E/V (RHE) (A)
(A
6 4 4
42 0
2
0.5
(B)
0.9 E/V (RHE)
Fig. 4. (A) Voltammogramof a Pt(lll) electrode with OTe = 0.24 in 0.5 M H2SO4. (B) Voltammogramof the same electrode in 0.5 M H 2 S O 4 d0.1 M HCOOH.
0
!
0.5 0.9 E/V (RHE)
(B)
coverages are reported in Fig. 7. The current values for high coverages are represented as open circles.
3.4. Poison formation experiments It has been shown that poison formation for low tellurium coverages is rather small during the voltammetric scan. This effect may be a consequence of either a decrease in the poison formation reaction rate or a complete inhibition of poison formation. In the first case, poison would still accumulate on the surface to a significant extent, but in a voltammetric scan at 50 mV s -1 the amount accumulated would be negligible. However, the general behaviour observed in the experiments resembles that found for bismuth- and arsenic-modified P t ( l l l ) electrodes [3,13], where an electronic effect was found [4]. In order to prove this hypothesis, long-term poison formation experiments were carried out at open circuit. Fig. 8 shows the results obtained for a tellurium coverage of 0.015. For the unmodified surface, the maximum amount of poison that can be accumulated corresponds to an oxidation charge (Qpi) of 1 9 2 / z C cm -2 [4]. In the presence of this very low amount of adsorbed tellurium, Qpi = 18 /zC cm -2, which is just one-tenth of the value obtained for the unmodified surface. Such an
Fig. 5. (A) Voltammogramof a Pt(111) electrode with OT~ = 0.30 in 0.5 M H 2 S O 4. (B) Voltammogramof the same electrode in 0.5 M H 2 S O 4 40.1 M HCOOH.
important inhibition cannot be explained through a simple third body mechanism, since the amount of adatoms on the surface is too low. It appears that the only possible mechanism is an electronic effect of the adsorbed adatom on the neighbouring P t ( l l 1) domains.
2
4 A.
0
0.5 0.9 E/V (RHE)
Fig. 6. Voltammogramof a Pt(111) electrode with Oxe ----0.4 in 0.5 M H 2 S O 4 4- 0.1 M HCOOH with partial desorptionof telluriumat the upper potential limit. Only positive-goingscans are shown. Arrows indicatethe evolution of the voltammetricprofile.
165
E. Herrero et al. /Journal of Electroanalytical Chemistry 394 (1995) 161-167
200
150 0
0
,?,
4
<
~,~ 100 gl.
3
..,..... °~
50
0 0.0
I
i
i
I
0.1
0.2
0.3
0.4
i
0.5
Or,
Fig. 7. Maximum current densities for the oxidation of formic acid obtained in the first positive-going scan vs. tellurium coverage in 0.5 M
0 0.00
0.01
0.02
0.03
0.04
0.05
Ox, Fig. 9. Maximum stripping charge of COad~ originating from poison formation reaction from formic acid.
U2SO 4.
Plotting the oxidation charge of the poison intermediate vs. the tellurium coverage (Fig. 9) leads to a very similar plot as obtained for arsenic and bismuth [4]. In the latter case, it has been estimated that every bismuth adatom impedes poison formation in all platinum atoms at a distance lower than six atoms from an adatom. The similarities between the two systems indicate that the tellurium adatoms on the surface reach a comparable number of platinum atoms. The general behaviour of tellurium against poison formation is comparable with that observed in the
50
d
<
-1
Fig. 8. Poison stripping experiment on Te + Pt(111) electrodes for formic acid in 0.5 M H2SO 4 with ~9.r¢= 0.02: (a) voltammogram of the electrode before the dissociative adsorption experiment; (b) adsorption profile of the electrode blocked by the poison; (c) poison stripping; (d) recovery of the adsorptive properties of the electrode after poison stripping (broken line).
case of bismuth and arsenic [4] and different from that reported in the case of selenium [9].
4. Discussion The four adatoms studied (Bi, As, Se and Te), when adsorbed on P t ( l l l ) , yield two different inhibition mechanisms for the poison formation reaction from formic acid, i.e. an important long-range electronic effect has been found for bismuth, arsenic [4] and tellurium at very low adatom coverages, whereas a short-range third body effect has been observed for selenium-modified electrodes, where the amount of poison decreases linearly with selenium coverage [9]. In the cases in which an electronic effect was found, it extended over a long distance and affected the platinum atoms within 6 - 7 rows around the adatom, which is probably randomly adsorbed on the P t ( l l l ) substrate at low coverages. Model calculations of the influence of an adatom on the Fermi level density of states indicates that the perturbation of an adatom extends further than the next-nearest neighbour to the adatom [16], but there is no indication that the influence could reach such a distance. In our case, the great extent of the inhibition effect may be the joint result of the modification of the electronic properties of the surface and its influence on the double layer properties. The modification of the double layer properties induced by the adsorbed adatom includes a change in the water adsorption modes and anion adsorption. The change in anion adsorption probably does not affect poison formation since it takes place in the low potential range where anions are desorbed [17]. This modification may extend further than the pure electronic effect if ice-like adsorbed
166
E. Herrero et aL /Journal of Electroanalytical Chemistry 394 (1995) 161-167
water layers are considered [18]. Thus a single adatom may disrupt the ordered water adlayer, and extend its effect over a long distance. In order to understand better the difference in the behaviour of the adatoms studied, a comparison of the electronic properties of the adatoms and platinum was performed. As a first approach, we compared the electronegativities of the adatoms and platinum. Tellurium has the same electronegativity as platinum (2.1), selenium has a higher electronegativity (2.4), and arsenic and bismuth have lower electronegativities (2 and 1.8 respectively) [19]. Taking into account the observed inhibition of the poison formation reaction, adatoms with a lower electronegativity than platinum give an electronic effect on poison formation and adatoms with a higher electronegativity than platinum give a third body effect. For tellurium, the behaviour is not clear. Electronegativities are assigned for non-condensed environments. Therefore it may be possible that the electronegativities do not reflect the electronic properties when the atoms are in the solid state. A better approach would be to consider the work function values. Selenium has a higher work function than platinum (5.9 vs. 5.6 eV) and the three other adatoms have a lower work function (4.22 eV for Bi, 4.95 eV for Te and 4.9-5.4 eV for As) [20]. These data agree well with the inhibition effects found for the different adatoms. Thus adatoms displaying a lower work function than platinum modify the properties of the surface in such a way that an important inhibition through an electronic effect is found. For the elements that have a higher work function than platinum, the modifications of the surface properties do not alter the behaviour of the surface with respect to CO formation from formic acid and only a third body effect is found. This qualitative approach does not take into account the work function of the P t ( l l l ) substrate (6.1 eV [21]), and is only based on the mean values obtained for polycrystalline materials. However, it is implicit in the approach that the work function of the adatoms parallels the behaviour observed for the substrate, i.e. a layer of the adatom on the P t ( l l l ) electrode will also have a higher work function than that obtained for the polycrystalline material. It is clear that more experiments with other adatoms under the same controlled conditions are needed to corroborate the relationship proposed between the inhibition mechanism for poison formation and the work function of the adatom. The difference from the behaviour obtained on the Pt(100) substrate, where the long-range order effect is not observed, is probably related to the different bonding of the adatom to the surface [22] and the ability of the surface to form a water adlayer. It is known that an ordered water adlayer is not formed on f.c.c. (100) surfaces in vacuum [18]. The absence of the ordered adlayer is probably linked to the faster poisoning of this latter electrode compared with the Pt(ll 1) electrode. The adatoms cannot disturb the water adlayer, since it is probably not formed on this
plane, and the effect of the adatoms is restricted only to the nearest neighbours (third body effect). It would also be interesting to find a correlation with the catalysis of direct formic acid oxidation. This is a more complicated issue since formic acid oxidation in the presence of adatoms does not always occur in the same potential range at different adatom coverages. However, a trend can be observed in the maximum currents obtained for formic acid oxidation: Se [9] < Te < As [3] < Bi [13]. Selenium has the highest work function, and the maximum current found is approximately the same as the intrinsic activity of platinum. At the other extreme, bismuth has the lowest work function and the highest catalytic activity. This provides a qualitative picture of the effects of the adatoms on the direct oxidation of formic acid, i.e. a higher catalytic activity is obtained for elements having a lower work function than platinum when adsorbed on P t ( l l l ) . Nevertheless, a quantitative approach needs a more detailed knowledge of how the adatom changes the energy levels of platinum and to what extent.
Acknowledgements This work was carried out within the framework of the DGICYT project PB 93-0944. J.M.F. acknowledges fruitful discussions with Dr. T. Iwasita. E.H. and M.J.L. acknowledge the award of grants by the Conselleria de Educaci6 i Ci~ncia of the Generalitat Valenciana and Fundaci6n C.A.M. respectively.
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